Add bindings doc for Sunplus SoC SP7021 PWM Driver
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Hammer Hsieh <hammerh0314@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>