mx31moboard: Add poweroff support

This patch add poweroff support on mx31moboard platform.
A watchdog timeout is generated while enabling the watchdog
reset IO to trigger a PMIC poweroff.

Signed-off-by: Philippe Rétornaz <philippe.retornaz@epfl.ch>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>