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

i.MX8QM/QXP supports inverted PWM output, thus #pwm-cells needs to be set
to 3.

Fixes: 23fa99b2

 ("arm64: dts: freescale: imx8-ss-lsio: add support for lsio_pwm0-3")
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>