Add configurable ramstage support for minimal PCI scanning

This CL has changes that allow us to enable a configurable
ramstage, and one change that allows us to minimize PCI
scanning. Minimal scanning is a frequently requested feature.

To enable it, we add two new variables to src/Kconfig
CONFIGURABLE_RAMSTAGE
is the overall variable controlling other options for minimizing the
ramstage.

MINIMAL_PCI_SCANNING is how we indicate we wish to enable minimal
PCI scanning.

Some devices must be scanned in all cases, such as 0:0.0.

To indicate which devices we must scan, we add a new mandatory
keyword to sconfig

It is used in place of on, off, or hidden, and indicates
a device is enabled and mandatory. Mandatory
devices are always scanned. When MINIMAL_PCI_SCANNING is enabled,
ONLY mandatory devices are scanned.

We further add support in src/device/pci_device.c to manage
both MINIMAL_PCI_SCANNING and mandatory devices.

Finally, to show how this works in practice, we add mandatory
keywords to 3 devices on the qemu-q35.

TEST=
1. This is tested and working on the qemu-q35 target.
2. On CML-Hatch

Before CL:
Total Boot time: ~685ms

After CL:
Total Boot time: ~615ms

Change-Id: I2073d9f8e9297c2b02530821ebb634ea2a5c758e
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36221
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jeremy Soller <jeremy@system76.com>

util/sconfig/sconfig.tab.h_shipped

@@ -1,8 +1,8 @@
-/* A Bison parser, made by GNU Bison 3.0.5.  */
+/* A Bison parser, made by GNU Bison 3.0.4.  */
 
 /* Bison interface for Yacc-like parsers in C
 
-   Copyright (C) 1984, 1989-1990, 2000-2015, 2018 Free Software Foundation, Inc.
+   Copyright (C) 1984, 1989-1990, 2000-2015 Free Software Foundation, Inc.
 
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -30,8 +30,8 @@
    This special exception was added by the Free Software Foundation in
    version 2.2 of Bison.  */
 
-#ifndef YY_YY_UTIL_SCONFIG_SCONFIG_TAB_H_SHIPPED_INCLUDED
-# define YY_YY_UTIL_SCONFIG_SCONFIG_TAB_H_SHIPPED_INCLUDED
+#ifndef YY_YY_HOME_RMINNICH_PROJECTS_LINUXBOOT_COREBOOTNERF_GITHUBCOREBOOT_UTIL_SCONFIG_SCONFIG_TAB_H_SHIPPED_INCLUDED
+# define YY_YY_HOME_RMINNICH_PROJECTS_LINUXBOOT_COREBOOTNERF_GITHUBCOREBOOT_UTIL_SCONFIG_SCONFIG_TAB_H_SHIPPED_INCLUDED
 /* Debug traces.  */
 #ifndef YYDEBUG
 # define YYDEBUG 0
@@ -49,34 +49,35 @@ extern int yydebug;
     DEVICE = 259,
     REGISTER = 260,
     BOOL = 261,
-    HIDDEN = 262,
-    BUS = 263,
-    RESOURCE = 264,
-    END = 265,
-    EQUALS = 266,
-    HEX = 267,
-    STRING = 268,
-    PCI = 269,
-    PNP = 270,
-    I2C = 271,
-    APIC = 272,
-    CPU_CLUSTER = 273,
-    CPU = 274,
-    DOMAIN = 275,
-    IRQ = 276,
-    DRQ = 277,
-    SLOT_DESC = 278,
-    IO = 279,
-    NUMBER = 280,
-    SUBSYSTEMID = 281,
-    INHERIT = 282,
-    IOAPIC_IRQ = 283,
-    IOAPIC = 284,
-    PCIINT = 285,
-    GENERIC = 286,
-    SPI = 287,
-    USB = 288,
-    MMIO = 289
+    STATUS = 262,
+    MANDATORY = 263,
+    BUS = 264,
+    RESOURCE = 265,
+    END = 266,
+    EQUALS = 267,
+    HEX = 268,
+    STRING = 269,
+    PCI = 270,
+    PNP = 271,
+    I2C = 272,
+    APIC = 273,
+    CPU_CLUSTER = 274,
+    CPU = 275,
+    DOMAIN = 276,
+    IRQ = 277,
+    DRQ = 278,
+    SLOT_DESC = 279,
+    IO = 280,
+    NUMBER = 281,
+    SUBSYSTEMID = 282,
+    INHERIT = 283,
+    IOAPIC_IRQ = 284,
+    IOAPIC = 285,
+    PCIINT = 286,
+    GENERIC = 287,
+    SPI = 288,
+    USB = 289,
+    MMIO = 290
   };
 #endif
 
@@ -105,4 +106,4 @@ extern YYSTYPE yylval;
 
 int yyparse (void);
 
-#endif /* !YY_YY_UTIL_SCONFIG_SCONFIG_TAB_H_SHIPPED_INCLUDED  */
+#endif /* !YY_YY_HOME_RMINNICH_PROJECTS_LINUXBOOT_COREBOOTNERF_GITHUBCOREBOOT_UTIL_SCONFIG_SCONFIG_TAB_H_SHIPPED_INCLUDED  */