soc/amd/picasso/psp_verstage: Implement get_uart_base

The Picasso PSP doesn't support mapping the UART, so add a dummy
function to return NULL.

BUG=b:215599230
TEST=Build and boot morphius

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Ie1f033ff86ebb0f755a9a0b6ff293aa3c8bbbeb6
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61608
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>

src/soc/amd/picasso/psp_verstage/Makefile.inc

@@ -5,6 +5,7 @@ verstage-generic-ccopts += -I$(src)/vendorcode/amd/fsp/picasso/include
 
 verstage-y += svc.c
 verstage-y += chipset.c
+verstage-y += uart.c
 
 verstage-y += $(top)/src/vendorcode/amd/fsp/picasso/bl_uapp/bl_uapp_startup.S
 verstage-y += $(top)/src/vendorcode/amd/fsp/picasso/bl_uapp/bl_uapp_end.S

src/soc/amd/picasso/psp_verstage/uart.c

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <bl_uapp/bl_syscall_public.h>
+#include <amdblocks/uart.h>
+#include <types.h>
+
+uintptr_t get_uart_base(unsigned int idx)
+{
+	/* Mapping the UART is not supported. */
+	return 0;
+}