intel/cpu: rename car.h to romstage.h

This header has nothing to do with cache-as-ram. Therefore, 'car'
is the wrong term to use. It is about providing a prototype for
*romstage*.

Change-Id: Ibc5bc6f3c38e74d6337c12f246846853ceae4743
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/6661
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>