ARM: dts: imx6: RDU2: disable internal watchdog

ARM: dts: imx6: RDU2: disable internal watchdog

The system has an external watchdog in the environment processor
so the internal watchdog is of no use.

Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: cphealy@gmail.com
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>