mb/google/rex/var/screebo: add FP_MCU fw_config

Add FP_MCU definitions for fw_config according to the current
build matrix.

BUG=b:291155207
TEST=emerge-rex coreboot

Change-Id: Id67b20a750d14eb23c62be9a30a5ef21d80e486a
Signed-off-by: YH Lin <yueherngl@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77597
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Subrata Banik <subratabanik@google.com>

src/mainboard/google/rex/variants/screebo/overridetree.cb

@@ -18,6 +18,10 @@ fw_config
 		option TOUCHSCREEN_ILITEK	1
 		option TOUCHSCREEN_ELAN		2
 	end
+	field FP_MCU 10
+		option FP_MCU_STM		0
+		option FP_MCU_NPCX		1
+	end
 	field DB_SD 11 12
 		option SD_GL9750		0
 		option SD_RTS5227S		1