dt-bindings: pwm: rockchip: Add pwm center-aligned optional property

Signed-off-by: Steven Liu <steven.liu@rock-chips.com>
Change-Id: Ic1b8c9e13d12bc730eab2564784c71eb0d0ce796

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

@@ -17,6 +17,9 @@ Required properties:
  - #pwm-cells: must be 2 (rk2928) or 3 (rk3288). See pwm.txt in this directory
    for a description of the cell format.
 
+Optional properties:
+ - center-aligned: support pwm output center aligned mode.
+
 Example:
 
 	pwm0: pwm@20030000 {
@@ -24,4 +27,5 @@ Example:
 		reg = <0x20030000 0x10>;
 		clocks = <&cru PCLK_PWM01>;
 		#pwm-cells = <2>;
+		center-aligned;
 	};