arm64: dt: imx8mq: support pwm polarity inversion

The i.MX8MQ has the same PWM IP as i.MX6 / i.MX7. This IP and the driver
supporting pwm polarity inversion. Switch CPU device tree fragment to
use 3 pwm-cells and correct board device trees.

Signed-off-by: Markus Niebel <Markus.Niebel@ew.tq-group.com>
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>