ARM: dts: imx7: use 3 PWM cells

The PWM driver has now capability to specify the PWM polarity
which is e.g. for backlight control. Allow to make use of PWM
polarity by specifying pwm-cells to be 3 in the base dt.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>