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>

Documentation/devicetree/bindings/pwm/pwm-rockchip.txt

@@ -3,7 +3,8 @@ Rockchip PWM controller
 Required properties:
  - compatible: should be "rockchip,<name>-pwm"
    "rockchip,rk2928-pwm": found on RK29XX,RK3066 and RK3188 SoCs
-   "rockchip,rk3288-pwm": found on RK3288 SoC
+   "rockchip,rk3288-pwm": found on RK3288 SOC
+   "rockchip,rv1108-pwm", "rockchip,rk3288-pwm": found on RV1108 SoC
    "rockchip,vop-pwm": found integrated in VOP on RK3288 SoC
  - reg: physical base address and length of the controller's registers
  - clocks: See ../clock/clock-bindings.txt