Get rid of AUTO_XIP_ROM_BASE

Get rid of AUTO_XIP_ROM_BASE

That value is now generated from a code address and CONFIG_XIP_ROM_SIZE.
This works as MTRRs are fully specified by their size and any address
within the range.

Change-Id: Id35d34eaf3be37f59cd2a968e3327d333ba71a34
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/348
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>