ARM: dts: imx51-zii-rdu1: Add node for RAVE SP device

With MFD and watchdog drivers for RAVE SP device support added by
538ee272 ("mfd: Add driver for RAVE Supervisory Processor") and
c3bb3334 ("watchdog: Add RAVE SP watchdog driver") add
corresponding DT node for RDU.

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