ARM: dts: imx6dl-prtvt7: fix PWM cell count for the backlight node.

At some point PWM cell count was changed, but it didn't triggered any
error, since this DT was overwriting "#pwm-cells".
To make sure, we are in sync with the kernel driver, remove this
property and fix the pwm consumer.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>