[MTD] maps/ixp4xx: half-word boundary and little-endian fixups

ixp4xx updates:
  - Handle reads that don't start on a half-word boundary.
  - Make it work when CPU is in little-endian mode.

Signed-off-by: John Bowler <jbowler@acm.org>
Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: David Vrabel <dvrabel@arcom.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>