mb/google/skyrim/var/frostflow: Override SPI flash bus speed

Add configuration to bump up the SPI flash bus speed from 66 MHz to 100
MHz starting the board version of the current phase.

BUG=b:260127676
TEST=Build and boot to OS in Frostflow with 100 MHz SPI bus speed.
Observe that the boot time improved by 100 ms compared to 66 MHz SPI
flash bus speed.

firmware log:
SPI fast read speed: 100 MHz
At 66 MHz:
Total Time: 1,563,384
At 100 MHz:
Total Time: 1,462,570

Change-Id: I9435f4ad0d3541b040703dc9a453badbd080dc09
Signed-off-by: Frank Wu <frank_wu@compal.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/72694
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

src/mainboard/google/skyrim/Kconfig

@@ -157,6 +157,7 @@ config OVERRIDE_EFS_SPI_SPEED
 config OVERRIDE_EFS_SPI_SPEED_MIN_BOARD
 	hex
 	default 0x5 if BOARD_GOOGLE_SKYRIM
+	default 0x1 if BOARD_GOOGLE_FROSTFLOW
 	default 0xffffffff
 	help
 	  Minimum board version starting which the Override EFS SPI Speed