Kconfig: Add new config for OVERRIDE_DEVICETREE

This change adds a new config option OVERRIDE_DEVICETREE that allows
variants to provide an override devicetree file to override the
registers and/or add new devices on top of the ones provided by
baseboard devicetree using CONFIG_DEVICETREE.

BUG=b:80081934

Change-Id: Ica046b7e0d70d0f1e8d94da714d1e62032277916
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/26691
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/Kconfig

@@ -295,6 +295,17 @@ config DEVICETREE
 	  Examples: "devicetree.variant.cb"
 	            "variant/devicetree.cb"
 
+config OVERRIDE_DEVICETREE
+	string
+	default ""
+	help
+	  This symbol allows variants to provide an override devicetree file to
+	  override the registers and/or add new devices on top of the ones
+	  provided by baseboard devicetree using CONFIG_DEVICETREE.
+
+	  Examples: "devicetree.variant-override.cb"
+	            "variant/devicetree-override.cb"
+
 config CBFS_SIZE
 	hex "Size of CBFS filesystem in ROM"
 	# Default value set at the end of the file