ACPI: Move PICM declaration

Variable PICM was not inside GNVS region and can use a static
initialisation value.

For most AMD platforms PICM default changes from 1 to 0.

Fix comments about PICM==0 used to indicate use of i8259 PIC for
interrupt delivery.

Change-Id: I525ef8353514ec32941c4d0c37cab38aa320cb20
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49905
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/acpi/dsdt_top.asl

@@ -11,5 +11,8 @@
 /* Operating system enumeration. */
 Name (OSYS, 0)
 
+/* Zero => PIC mode, One => APIC Mode */
+Name (PICM, Zero)
+
 /* Power state (AC = 1) */
 Name (PWRS, One)