mb/google/zork/berknip: Increase eMMC initial clock frequency

This will reduce boot time by 7ms. Some of the initial designs
don't have a pull-up resistor on the CMD line. These designs still boot
at 400 kHz despite not having the pull-up.

BUG=b:158766134
TEST=Boot Berknip w/ eMMC to OS.
BRANCH=zork

Change-Id: I5d55f55b8208b4dc3fbdc9d1ec6333f9e211e3fd
Signed-off-by: Rob Barnes <robbarnes@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45931
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/zork/variants/berknip/overridetree.cb

@@ -38,6 +38,12 @@ chip soc/amd/picasso
 		.early_init = true,
 	}"
 
+	register "emmc_config" = "{
+		.timing = SD_EMMC_EMMC_HS400,
+		.sdr104_hs400_driver_strength = SD_EMMC_DRIVE_STRENGTH_A,
+		.init_khz_preset = 400,
+	}"
+
 	# See AMD 55570-B1 Table 13: PCI Device ID Assignments.
 	device domain 0 on
 		subsystemid 0x1022 0x1510 inherit