dt-bindings: ap806: document gpio marvell,pwm-offset property

Update the example as well. Add the '#pwm-cells' and 'clocks' properties
for a complete working example.

Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Baruch Siach <[e-mail]>
Signed-off-by: Bartosz Golaszewski <[e-mail]>

Documentation/devicetree/bindings/arm/marvell/ap80x-system-controller.txt

···
 
 - offset: offset address inside the syscon block
 
+Optional properties:
+
+- marvell,pwm-offset: offset address of PWM duration control registers inside
+  the syscon block
+
 Example:
 ap_syscon: system-controller@6f4000 {
 	compatible = "syscon", "simple-mfd";
···
 		gpio-controller;
 		#gpio-cells = <2>;
 		gpio-ranges = <&ap_pinctrl 0 0 19>;
+		marvell,pwm-offset = <0x10c0>;
+		#pwm-cells = <2>;
+		clocks = <&ap_clk 3>;
 	};
 };