mb/google/skyrim: Enable amdfw separation

Select the config to separate the AMDFW binary from the verified boot
section.

BUG=b:203597980
TEST=Build Skyrim BIOS image and boot to OS with PSP verstage passing
the hash table and PSP verifying the binaries against the hash table.
Observe boot time improvement of ~120 ms while operating  SPI bus at 66
MHz with PSP verstage enabled.
Before this patch series:
 508:finished loading body             1,978,053,432 (201,518)
After this patch series:
 508:finished loading body             7,948,797,849 (83,460)

Change-Id: I78ec6d28b4c5fc40bdade47489d58180a54dee4d
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67261
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jon Murphy <jpmurphy@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>

src/mainboard/google/skyrim/Kconfig

@@ -97,6 +97,7 @@ config VBOOT
 config VBOOT_STARTS_BEFORE_BOOTBLOCK
 	bool "Enable PSP_verstage"
 	default y if VBOOT
+	select SEPARATE_SIGNED_PSPFW
 
 config VBOOT_STARTS_IN_BOOTBLOCK
 	bool

src/mainboard/google/skyrim/chromeos.fmd

@@ -4,11 +4,13 @@ FLASH@0xFF000000 16M {
 		RW_SECTION_A 3M {
 			VBLOCK_A 8K
 			FW_MAIN_A(CBFS)
+			SIGNED_AMDFW_A 1536K
 			RW_FWID_A 256
 		}
 		RW_SECTION_B 3M {
 			VBLOCK_B 8K
 			FW_MAIN_B(CBFS)
+			SIGNED_AMDFW_B 1536K
 			RW_FWID_B 256
 		}
 		RW_ELOG(PRESERVE) 4K