drivers: pwm_nrfx: Connect IRQ handlers for anomaly 109 workaround

The workaround for the nRF52 anomaly 109 that is implemented in
the nrfx_pwm driver uses interrupts generated by a selected EGU
instance and by the enabled PWM instances (even if the interrupts
are not used in generation of the PWM output signals).
Add required IRQ_CONNECT calls so that those interrupts are
properly handled.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>