dt-bindings: pwm: Document clk based PWM controller

Add YAML devicetree binding for clk based PWM controller

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nikita Travkin <nikita@trvn.ru>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>