drivers: pwm_nrf5_sw: Use GPIOTE SET and CLR tasks when available

When possible, use separate GPIOTE tasks for setting the PWM output
high and low instead of using one task to toggle it. This is crucial
for DPPI where the same task cannot be used in more than one channel.

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