soc/intel/elkhartlake: Define DIMM_SPD_SIZE in SoC Kconfig

The default SPD size is set to 256 bytes, instead of 512 for
LPDDR4/DDR4 if not overridden by the mainboard Kconfig. This caused
the SMBus libraries to read only the lower half of the DIMM SPD on
protectli/vault_ehl. The lower half of the SPD passed to FSP causes
a bug in DIMM change detection, which relies on the CRC of the
manufacturer bytes in the upper half of the SPD (CRC of zero bytes
always gives zero so no change was assumed). Setting the DIMM SPD size
to 512 fixes it.

Setting the SPD size in SoC will also avoid such problems in the future
Elkhart Lake ports. Elkhart Lake supports only LPDDR4/DDR4 so providing
the correct default of 512 bytes is an obvious thing to do.

TEST=Boot Protectli VP2420 (vault_ehl) with different DIMMs and see
FSP is retraining the memory instead of doing the fastboot with old
DIMM data.

Signed-off-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Change-Id: I998ed8781951034419cadc26c04ff1e0a124b267
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73933
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Mario Scheithauer <mario.scheithauer@siemens.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/siemens/mc_ehl/Kconfig

@@ -31,9 +31,6 @@ config MAINBOARD_PART_NUMBER
 config DEVICETREE
 	default "variants/\$(CONFIG_VARIANT_DIR)/devicetree.cb"
 
-config DIMM_SPD_SIZE
-	default 512
-
 config SOC_INTEL_ELKHARTLAKE_TCO_NO_REBOOT_EN
 	default y
 

src/soc/intel/elkhartlake/Kconfig

@@ -272,4 +272,8 @@ config SOC_INTEL_ELKHARTLAKE_TCO_NO_REBOOT_EN
 	  for the second time. Since this feature is not exposed to the OS in the
 	  standard TCO interface, this setting can be enabled on firmware level.
 	  This might be useful depending on the TCO policy.
+
+config DIMM_SPD_SIZE
+	default 512
+
 endif