drivers: pwm: nrfx: Improve runtime PM

Rework PM handling to use pm_device_driver_init(). Shim is not using
put and get internally as there is no api that disables or stops
pwm so it is hard to determine when to put the device. There are cases
when PWM peripheral is stopped but PWM is still active because
duty cycle is 100% or 0% and pin is driven by GPIO and not PWM.

If user want to use runtime PM with PWM it is possible and getting
the device will initialize internal data and putting will suspend
by forcing PWM stop if used and setting pins to sleep state. However,
from power consumption perspective it is enough to set 0% or 100%
duty cycle on all channels.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>