dt-bindings: input: convert zii,rave-sp-pwrbutton.txt to yaml

Convert device tree binding doc zii,rave-sp-pwrbutton.txt to yaml format.
Additional changes:
- add ref to input.yaml.
- remove mfd node in example.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20241010-zii_yaml-v2-1-0ab730607422@nxp.com


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>