mb/google/fatcat: Set SkipExtGfxScan FSP-M UPD

This patch overrides `SkipExtGfxScan` UPD as the Fatcat device is
equipped with an on-board graphics device hence, skip scanning
external GFX devices.

TEST=Able to save ~10ms+ boot time on google/fatcat.

FSP FPDT Data is showing the timestamp between those function calls.

Without this patch:
  50b8	680462	42	76f18bda-2195-4fb6-9a940e0bacdeecab
  50b9	696649	16187	76f18bda-2195-4fb6-9a940e0bacdeecab

With this patch:

    `CheckOffboardPcieVga` is not getting called.

Change-Id: I198a99ac5596ff98a9cc673dbd84889d7c5386cb
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/88888
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Pranava Y N <pranavayn@google.com>

src/mainboard/google/fatcat/variants/baseboard/fatcat/devicetree.cb

@@ -101,6 +101,9 @@ chip soc/intel/pantherlake
 		[VR_DOMAIN_SA] = 38 * 4
 	}"
 
+	# Set on-board graphics as primary display
+	register "skip_ext_gfx_scan" = "true"
+
 	# Enable CNVi BT
 	register "cnvi_bt_core" = "true"