dt-bindings: watchdog: add mpc8xxx-wdt support

Add description of DT bindings for mpc8xxx-wdt driver which
handles the CPU watchdog timer on the mpc83xx, mpc86xx and mpc8xx.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>