dt-bindings: pwm: Convert pwm-rockchip.txt to YAML

Current dts files with 'pwm' nodes are manually verified. In order to
automate this process pwm-rockchip.txt has to be converted to YAML.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>