pwm: Autodetect default value for of_pwm_n_cells from device tree

This allows to simplify all drivers that use three pwm-cells.
The only ugly side effect is that if a driver specified of_pwm_n_cells = 2
it suddenly supports device trees that use #pwm-cells = <3>. This however
isn't a bad thing because the driver doesn't need explicit support for
three cells as the core handles all the details. Also there is no such
in-tree driver.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>