dt-bindings: backlight: Add binding for RAVE SP backlight driver

Add Device Tree bindings for RAVE SP backlight driver - an MFD cell of
parent RAVE SP driver (documented in
Documentation/devicetree/bindings/mfd/zii,rave-sp.txt).

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>