intel/i945,gm45,pineview,x4x: Move stage cache support function

Let garbage-collection take care of stage_cache_external_region()
when it is not needed and move implementation to a suitable file
already building for needed stages.

Change-Id: Ic32adcc62c7ee21bf38e2e4e5ece00524871b091
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34670
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/northbridge/intel/pineview/Makefile.inc

@@ -30,8 +30,5 @@ romstage-y += raminit.c
 romstage-y += early_init.c
 
 postcar-y += ram_calc.c
-romstage-$(CONFIG_CACHE_RELOCATED_RAMSTAGE_OUTSIDE_CBMEM) += stage_cache.c
-ramstage-$(CONFIG_CACHE_RELOCATED_RAMSTAGE_OUTSIDE_CBMEM) += stage_cache.c
-postcar-$(CONFIG_CACHE_RELOCATED_RAMSTAGE_OUTSIDE_CBMEM) += stage_cache.c
 
 endif