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>