soc/intel/elkhartlake: add __weak to mainboard_get_dram_part_num()

Add missing __weak declaration to Elkhart Lake's definition of
mainboard_get_dram_part_num().

BUG=b:169774482, b:168724473
TEST=none

Change-Id: I883d662315a9ab91eb6183ebcdfd7e7cc8064c64
Signed-off-by: Nick Vaccaro <nvaccaro@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45871
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/elkhartlake/romstage/romstage.c

@@ -21,7 +21,7 @@
 	0x8d, 0x09, 0x11, 0xcf, 0x8b, 0x9f, 0x03, 0x23	\
 }
 
-bool mainboard_get_dram_part_num(const char **part_num, size_t *len)
+bool __weak mainboard_get_dram_part_num(const char **part_num, size_t *len)
 {
 	/* Default implementation, no need to override part number. */
 	return false;