samples: blinky_pwm: Use nRF SW PWM driver on nRF DKs

The default nRF hardware PWM driver is not very suitable for this
sample as that driver does not allow setting a period longer than
~262 ms. Use the nRF SW PWM driver instead. This will also ensure
that the driver is built by CI for nRF52, nRF53, and nRF91 Series.

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