nb/intel/pineview: Move the boilerplate mainboard_romstage_entry

The mainboard_romstage_entry function is mostly boilerplate, so move
it to a common location and provide mainboard specific callbacks.

Change-Id: I33cf1d6a60d272f490f41205ec725dee8b00242b
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/30851
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/northbridge/intel/pineview/Makefile.inc

@@ -21,6 +21,7 @@ ramstage-y += northbridge.c
 ramstage-y += gma.c
 ramstage-y += acpi.c
 
+romstage-y += romstage.c
 romstage-y += ram_calc.c
 romstage-y += raminit.c
 romstage-y += early_init.c