sconfig: Allow chipset to provide a base devicetree

This change extends the devicetree override one more layer and allows
the chipset to provide the base devicetree.  This allows the chipset to
assign alias names to devices as well as set default register values.
This works for both the baseboard devicetree.cb as well as variant
overridetree.cb.

chipset.cb:
device pci 15.0 alias i2c0 off end

devicetree.cb:
device ref i2c0 on end

BUG=b:156957424

Change-Id: Ia7500a62f6211243b519424ef3834b9e7615e2fd
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44037
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/Kconfig

@@ -536,6 +536,17 @@ source "src/vendorcode/*/Kconfig"
 
 source "src/arch/*/Kconfig"
 
+config CHIPSET_DEVICETREE
+	string
+	default ""
+	help
+	  This symbol allows a chipset to provide a set of default settings in
+	  a devicetree which are common to all mainboards. This may include
+	  devices (including alias names), chip drivers, register settings,
+	  and others. This path is relative to the src/ directory.
+
+	  Example: "chipset.cb"
+
 endmenu
 
 source "src/device/Kconfig"