arm64: dts: freescale: imx8-ss-dma: Fix #pwm-cells

i.MX8QXP supports inverted PWM output, thus #pwm-cells needs to be set
to 3.

Fixes: f1d6a6b9 ("arm64: dts: imx8qxp: add adma_pwm in adma")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>