Documentation: devicetree: Add Richtek RT9455 bindings

Signed-off-by: Anda-Maria Nicolae <anda-maria.nicolae@intel.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
[added default values to optional properties]
Signed-off-by: Sebastian Reichel <sre@kernel.org>