x86: mtrr: add CONFIG_CACHE_ROM support

x86: mtrr: add CONFIG_CACHE_ROM support

The CONFIG_CACHE_ROM support in the MTRR code allocates an MTRR
specifically for setting up write-protect cachine of the ROM. It is
assumed that CONFIG_ROM_SIZE is the size of the ROM and the whole
area should be cached just under 4GiB. If enabled, the MTRR code
will allocate but not enable rom caching. It is up to the callers
of the MTRR code to explicitly enable (and disable afterwards) through
the use of 2 new functions:
- x86_mtrr_enable_rom_caching()
- x86_mtrr_disable_rom_caching()

Additionally, the CACHE_ROM option is exposed to the config menu so
that it is not just selected by the chipset or board. The reasoning
is that through a multitude of options CACHE_ROM may not be appropriate
for enabling.

Change-Id: I4483df850f442bdcef969ffeaf7608ed70b88085
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/2918
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>