drivers: pwm: nrfx: Disable PWM peripheral when not used

Shim was not correctly disabling PWM when it was not used. Task
STOP was triggered but PWM->ENABLE remained set which caused
increased current. Added interrupt and enabled event handler in
the nrfx driver to allow disabling PWM on STOPPED event.

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