pwm: rockchip: document RK3288 SoC compatible

Document new compatible for PWM founding on RK3288 SoC
Signed-off-by: NCaesar Wang <caesar.wang@rock-chips.com>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>

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

 Rockchip PWM controller
 
 Required properties:
- - compatible: should be "rockchip,rk2928-pwm"
+ - compatible: should be "rockchip,<name>-pwm"
+   "rockchip,rk2928-pwm": found on RK29XX,RK3066 and RK3188 SoCs
+   "rockchip,rk3288-pwm": found on RK3288 SoC
+   "rockchip,vop-pwm": found integrated in VOP on RK3288 SoC
  - reg: physical base address and length of the controller's registers
  - clocks: phandle and clock specifier of the PWM reference clock
  - #pwm-cells: should be 2. See pwm.txt in this directory for a