[WATCHDOG] mpc8xxx_wdt: add support for MPC8xx watchdogs

The mpc8xxx_wdt driver is using two registers: SWSRR to push magic
numbers, and SWCRR to control the watchdog.  Both registers are available
on the MPC8xx, and seem to have the same offsets and semantics as in
MPC83xx/MPC86xx watchdogs.  The only difference is prescale value.  So
this driver simply works on the MPC8xx CPUs.

One quirk is needed for the MPC8xx, though.  It has small prescale value
and slow CPU, so the watchdog resets board prior to the driver has time to
load.  To solve this we should split initialization in two steps: start
ping the watchdog early, and register the watchdog userspace interface
later.

MPC823 seem to be the first CPU in MPC8xx line, so we use fsl,mpc823-wdt
compatible matching.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Tested-by: Jochen Friedrich <jochen@scram.de>
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>