The binding dictates using 3 pwm-cells. Adhere to that.

This fixes the following dtbs_check warning:

	arch/arm/boot/dts/nxp/imx/imx6sx-nitrogen6sx.dtb: pwm@208c000: #pwm-cells:0:0: 3 was expected
		from schema : http://devicetree.org/schemas/pwm/imx-pwm.yaml#

There is no need for an explicit status = "okay" in the pwm node as
the soc dtsi doesn't disable this device. Drop this property, too.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>