drivers: pwm: nrf: Use HW PWM by default when available

Enable HW PWM driver instead of the SW one on nRF SoCs where the PWM
peripheral is present.
Default PWM instances are also enabled on Nordic DK boards so that it
is possible to build the basic fade_led sample for them without extra
adjustments.

After the above changes are applied, some configuration alterations
in basic samples blink_led and fade_led become no longer needed.
These are removed. And the blink_led sample is corrected so that it
works with the nRF HW PWM driver as well.

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