ARM: imx27: add missing #pwm-cells property

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

Add the missing property.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>