soc: move mainboard_get_dram_part_num prototype to memory_info.h

BUG=b:169774661, b:168724473
TEST="emerge-volteer coreboot && emerge-nocturne coreboot &&
emerge-dedede coreboot" and verify they build successfully.

Change-Id: I8b228475621ca1035fe13f8311355fc3b926e897
Signed-off-by: Nick Vaccaro <nvaccaro@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45879
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/include/memory_info.h

@@ -108,4 +108,10 @@ struct memory_info {
 	struct dimm_info dimm[DIMM_INFO_TOTAL];
 } __packed;
 
+/*
+ * mainboard_get_dram_part_num returns a DRAM part number override string
+ *  return NULL = no part number override provided by mainboard
+ *  return non-NULL = pointer to a string terminating in '\0'
+ */
+const char *mainboard_get_dram_part_num(void);
 #endif