dt-bindings: pwm: imx-pwm: Unify #pwm-cells for all compatibles

Use #pwm-cells for all i.MX variants. Only fsl,imx1-pwm does not support
inverted PWM output. Keep it the same for consistency.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>