dt-bindings: pwm: Add description for rv1108 PWM

Add device tree bindings document for PWM on Rockchip rv1108 SoC.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>