mb/google/volteer: Add overridetree.cb for volteer variant

Instead of only using the baseboard devicetree add a placeholder
overridetree for volteer and refer to it in Kconfig.

This will allow us to add the volteer specific devices here instead
of at the baseboard level.

BUG=b:146482091

Signed-off-by: Duncan Laurie <dlaurie@google.com>
Change-Id: I7788a5473fc2275a9791fb27e0e4018a0efcd0f5
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40893
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/volteer/Kconfig

@@ -42,7 +42,7 @@ config DEVICETREE
 
 config OVERRIDE_DEVICETREE
 	string
-	default "variants/$(CONFIG_VARIANT_DIR)/overridetree.cb" if !BOARD_GOOGLE_VOLTEER
+	default "variants/$(CONFIG_VARIANT_DIR)/overridetree.cb"
 
 config DRIVER_TPM_SPI_BUS
 	default 0x1

src/mainboard/google/volteer/variants/volteer/overridetree.cb

@@ -0,0 +1,4 @@
+chip soc/intel/tigerlake
+	device domain 0 on
+	end
+end