dt-bindings: pwm: rockchip: Include generic PWM schema

Include generic pwm.yaml schema, which enforces PWM node naming and
brings pwm-cells requirement.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>