soc/amd/mendocino: Add svc_write_postcode call instead of stub

To assist in debugging, add a way for PSP_verstage to send postcodes to
the system.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I22e45e26f599a0b4f0b781e9b97fccb68e2e5cc1
Reviewed-on: https://review.coreboot.org/c/coreboot/+/71852
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/vendorcode/amd/fsp/mendocino/include/bl_uapp/bl_syscall_public.h

@@ -57,6 +57,7 @@ enum verstage_cmd_id {
 	CMD_SET_FW_HASH_TABLE,
 	CMD_GET_PREV_BOOT_STATUS,
 	CMD_GET_HSP_SECURE_STATE,
+	CMD_WRITE_POSTCODE,
 };
 
 struct mod_exp_params {