mb/google/dedede/var/metaknight: Set core display clock to 172.8 MHz

When using the default initial core display clock frequency, Metaknight
has a rare stability issue where the startup of Chrome OS in secure mode
may hang. Slowing the initial core display clock frequency down to
172.8 MHz as per Intel recommendation avoids this problem.

The CdClock=0xff is set in dedede baseboard,and we overwrite it as 0x0
(172.8 MHz) for metaknight.

BUG=None
BRANCH=dedede
TEST=Build firmware and verify on fail DUTs.
     Check the DUTs can boot up in secure mode well.

Change-Id: I987277fec2656fe6f10827bc6685d3d04093235e
Signed-off-by: David Wu <david_wu@quanta.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61327
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Ren Kuo <ren.kuo@quanta.corp-partner.google.com>

src/mainboard/google/dedede/variants/metaknight/overridetree.cb

@@ -69,6 +69,9 @@ chip soc/intel/jasperlake
 		.tdp_pl2_override = 12,
 	}"
 
+	# Core Display Clock Frequency selection
+	register "cd_clock" = "CD_CLOCK_172_8_MHZ"
+
 	device domain 0 on
 		device pci 04.0 on
 			chip drivers/intel/dptf