mb/protectli/vault_ehl: Set DIMM_MAX to 1

VP2420 (vault_ehl) has only 1 DIMM slot present. Set the DIMM_MAX to 1
to optimize the common libraries to not attempt to read and parse more
SPD than needed.

TEST=Boot Protectli VP2420 (vault_ehl) with different DIMMs and see
FSP is retraining the memory properly and fastboot is working.

Signed-off-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Change-Id: I29a99f387ffe2df1060547e0818c5c5b66a27061
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73819
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Michał Kopeć <michal.kopec@3mdeb.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/protectli/vault_ehl/Kconfig

@@ -29,6 +29,9 @@ config MAX_CPUS
 config CBFS_SIZE
 	default 0x900000
 
+config DIMM_MAX
+	default 1
+
 config VBOOT
 	select GBB_FLAG_DISABLE_EC_SOFTWARE_SYNC
 	select GBB_FLAG_DISABLE_FWMP