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/soc/intel/tigerlake/include/soc/romstage.h

@@ -5,8 +5,6 @@
 
 #include <fsp/api.h>
 
-/* Provide a callback to allow mainboard to override the DRAM part number. */
-const char *mainboard_get_dram_part_num(void);
 void mainboard_memory_init_params(FSPM_UPD *mupd);
 void systemagent_early_init(void);
 void romstage_pch_init(void);