drivers: pwm: nrfx: Improve device generation macros

Use NRFX_FOREACH_PRESENT macro to iterate over all PWM instances
and create device only for those enabled in the devicetree.
This approach removes need of changing driver code when new
instance id is added.

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