Rename the driver because now we support some MPC86xx processors.

There are no changes to the mpc83xx_wdt.c file, yet.  When possible, we do
file renames and changes separately (because Linus once asked so, because
it helps git to track the renamed files).
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Cc: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>