arm64: dt: imx8mn: support pwm polarity inversion

The i.MX8M Nano 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.

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>