pwm: rockchip: Add APB and function both clocks support

New PWM module provides two individual clocks for APB clock and function
clock.

Signed-off-by: David Wu <david.wu@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

@@ -6,7 +6,13 @@ Required properties:
    "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
+ - clocks: See ../clock/clock-bindings.txt
+   - For older hardware (rk2928, rk3066, rk3188, rk3228, rk3288, rk3399):
+     - There is one clock that's used both to derive the functional clock
+       for the device and as the bus clock.
+   - For newer hardware (rk3328 and future socs): specified by name
+     - "pwm": This is used to derive the functional clock.
+     - "pclk": This is the APB bus clock.
  - #pwm-cells: must be 2 (rk2928) or 3 (rk3288). See pwm.txt in this directory
    for a description of the cell format.