ARM: dts: imx: default to #pwm-cells = <3> in the SoC dtsi files

The imx-pwm driver supports 3 cells and this is the more flexible setting.
So use it by default and overwrite it back to two for the files that
reference the PWMs with just 2 cells to minimize changes.

This allows to drop explicit setting to 3 cells for the boards that already
depend on this. The boards that are now using 2 cells explicitly can be
converted to 3 individually.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>