83xx: Use the proper sequence for updating IMMR.

This ensures that subsequent accesses properly hit the new window.

The dcbi during the NAND loop was accidentally working around this;
it's no longer necessary, as the cache is not enabled.

Reported-by: Suchit Lepcha <Suchit.Lepcha@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>