dt-bindings: backlight: Convert zii,rave-sp-backlight.txt to yaml

Convert device tree binding doc zii,rave-sp-backlight.txt to yaml format.
Additional Changes:
- Remove mfd parent node at example.
- Ref to backlight's common.yaml

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20241010-zii_yaml-v2-2-0ab730607422@nxp.com


Signed-off-by: Lee Jones <lee@kernel.org>