intel/baytrail,broadwell: 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: Ia6adcc0c8bf6d4abc095ac669aaae876b33ed0f3
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34669
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/baytrail/Makefile.inc

@@ -13,7 +13,6 @@ romstage-y += iosf.c
 romstage-y += memmap.c
 romstage-y += pmutil.c
 romstage-y += spi.c
-romstage-y += stage_cache.c
 romstage-y += tsc_freq.c
 
 postcar-y += iosf.c
@@ -45,7 +44,6 @@ ramstage-y += sd.c
 ramstage-y += smm.c
 ramstage-y += southcluster.c
 ramstage-y += spi.c
-ramstage-y += stage_cache.c
 ramstage-y += tsc_freq.c
 ramstage-y += xhci.c
 ramstage-$(CONFIG_ELOG) += elog.c