MX31: Disable watchdog during low-power modes

Turn on the watchdog WDZST bit so that watchdog timer does not count during low power modes.

Prior to applying this patch mx31pdk board got watchdog resets because when it booted in the Linux prompt
and there was no activity, the system entered into idle mode while watchdog timer was still active.

Fix this by disabling watchdog timer during idle mode.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>