mb/google/guybrush/var/nipperkin: Override SPI fast speed

After assessing the signal integrity, 100 MHz SPI fast speed can be
enabled for SPI ROM.

BUG=None
TEST=Build and boot to OS in Nipperkin board version 2. Perform 250
iterations of warm and cold reset each.

Signed-off-by: Karthikeyan Ramasubramanian <kramasub@google.com>
Change-Id: Id973acb939b69e0beda26252e57a278892f2f57d
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59983
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Jon Murphy <jpmurphy@google.com>
Reviewed-by: Rob Barnes <robbarnes@google.com>

src/mainboard/google/guybrush/Kconfig

@@ -123,6 +123,7 @@ config OVERRIDE_EFS_SPI_SPEED
 config OVERRIDE_EFS_SPI_SPEED_MIN_BOARD
 	hex
 	default 0x4 if BOARD_GOOGLE_GUYBRUSH
+	default 0x2 if BOARD_GOOGLE_NIPPERKIN
 	default 0xffffffff
 	help
 	  Minimum board version starting which the Override EFS SPI Speed