Remove XIP_ROM_BASE

The base is now calculated automatically, and all mentions of that
config option were typical anyway (4GB - XIP_ROM_SIZE).

Change-Id: Icdf908dc043719f3810f7b5b85ad9938f362ea40
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/366
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>