ARM: dts: imx25: Add #pwm-cells to pwm4

The property '#pwm-cells' is currently missing. It is not possible to
use pwm4 without this property.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Fixes: 5658a68f ("ARM i.MX25: Add devicetree")
Cc: <stable@vger.kernel.org>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>