mb/google/skywalker: Add storage types to fw_config

Use the storage type from fw_config to determine which types of storage
need to be set up in the payload.

BUG=b:379008996
BRANCH=none
TEST=input "cbi set 6 0x40000000 4" in ec console, and see the
following log:

fw_config match found: STORAGE=STORAGE_UFS2X
storage_setup: eMMC: no, UFS: yes

Signed-off-by: Vince Liu <vince-wl.liu@mediatek.corp-partner.google.com>
Change-Id: Ieada3c56b0f69cc1ea3dab4e64641bfc2ba2a0fe
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87923
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Yidi Lin <yidilin@google.com>

src/mainboard/google/skywalker/devicetree.cb

@@ -10,4 +10,10 @@ fw_config
 		option AMP_RT1019 1
 		option AMP_ALC5645 3
 	end
+
+	field STORAGE 30 31
+		option STORAGE_UFS3X 0
+		option STORAGE_UFS2X 1
+		option STORAGE_EMMC 2
+	end
 end