soc/amd/stoneyridge/memmap: fix bug in bert_reserved_region

Changing the local pointer "start" has no effect.
Changing the value it points to has.

Change-Id: I1b689896fcf255b795b27d7a7163849d6dfdb00e
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40506
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/soc/amd/stoneyridge/memmap.c

@@ -32,7 +32,7 @@ void bert_reserved_region(void **start, size_t *size)
 	if (CONFIG(ACPI_BERT))
 		*start = cbmem_top();
 	else
-		start = NULL;
+		*start = NULL;
 	*size = BERT_REGION_MAX_SIZE;
 }