ARM: dts: imx51-zii-rdu1: Disable internal watchdog

imx51-zii-rdu1 has an external watchdog in the environment
microcontroller, so disable the internal one.

This aligns with what was done in commit 7055f714 ("ARM: dts:
imx6: RDU2: disable internal watchdog") for imx6 rdu2 board.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>