arm64: dts: renesas: r8a77965: Add #pwm-cells property

Add "#pwm-cells" property to "pwm@e6e31000" device node.

This silences the following DTC compiler warning:
Warning (pwms_property): Missing property '#pwm-cells' in node
/soc/pwm@e6e31000 or bad phandle (referred from /backlight:pwms[0])
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm64/boot/dts/renesas/r8a77965.dtsi

@@ -668,6 +668,7 @@ pwm0: pwm@e6e30000 {
 
 		pwm1: pwm@e6e31000 {
 			reg = <0 0xe6e31000 0 8>;
+			#pwm-cells = <2>;
 			/* placeholder */
 		};