Intel cpus: cache actual size of the Flash ROM device

Intel cpus: cache actual size of the Flash ROM device

Cache was enabled for the last 4 MB below 4 GB when ramstage is
loaded. This does not cover the case of a 8 MB Flash and could
overlap with some system device placed at high memory.

Use the actual device size for the cache region. Mainboard
may override this with Kconfig CACHE_ROM_SIZE if necessary.

Change-Id: I622223b1e2af0b3c1831f3570b74eacfde7189dc
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/641
Tested-by: build bot (Jenkins)
Reviewed-by: Idwer Vollering <vidwer@gmail.com>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>