nb/intel/nehalem: Move romstage boilerplate to a common location

Move the mainboard_romstage_entry to a common location and provide
mainboard specific callbacks.

Change-Id: Ia827053617cead5d2cf8e9f06cb68c2cbb668ca9
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35771
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
diff --git a/src/northbridge/intel/nehalem/Makefile.inc b/src/northbridge/intel/nehalem/Makefile.inc
index 180cc09..3b12bb5 100644
--- a/src/northbridge/intel/nehalem/Makefile.inc
+++ b/src/northbridge/intel/nehalem/Makefile.inc
@@ -26,6 +26,7 @@
 romstage-y += raminit.c
 romstage-y += raminit_tables.c
 romstage-y += early_init.c
+romstage-y += romstage.c
 
 smm-y += finalize.c