drivers: pwm_nrf5_sw: Extend to work also with DPPI

Instead of directly configuring PPI channels, use the GPPI helper
provided by nrfx. This allows using the driver on nRF53 an nRF91
Series where DPPI is available instead of PPI.

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