ARM: dts: imx6qdl-zii-rdu2: Populate RAVE SP backlight node

RAVE SP found on RDU2 implements backlight control compatible with the
rave-sp-backlight driver. Add a node to make it availible.

Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: cphealy@gmail.com
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
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>