drivers: watchdog: add a DM driver for the MPC8xx watchdog

drivers: watchdog: add a DM driver for the MPC8xx watchdog

This patch adds a DM driver for the MPC8xx watchdog.
Basically, the watchdog is enabled by default from the start and
SYPCR register has to be writen once to set the timeout and/or
deactivate the watchdog. Once written, it cannot be written again.

It means that wdt_stop() can be called before wdt_start() to stop the
watchdog, but cannot be called if wdt_start() has been called.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>