The pwm-node is missing its #pwm-cells property. The pwm-framework will
complain about this.

Add the missing property.
Signed-off-by: NSteffen Trumtrar <s.trumtrar@pengutronix.de>
Acked-by: NSascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>