ramips: add #pwm-cells property to MT76x8 dts

To be able to configure pwms the pwm driver needs to know the number off
cells in the "pwms" property. For this platform this is 2.

Signed-off-by: Micke Prag <micke.prag@telldus.se>

target/linux/ramips/dts/mt7628an.dtsi

@@ -232,6 +232,7 @@
 		pwm: pwm@5000 {
 			compatible = "mediatek,mt7628-pwm";
 			reg = <0x5000 0x1000>;
+			#pwm-cells = <2>;
 
 			resets = <&rstctrl 31>;
 			reset-names = "pwm";