boards: nrf: Add pwm-led0 alias in all Nordic DKs with enabled PWM node

The `pwm-led0` alias is required for building fade_led and blink_led
samples. Add a suitable `pwmleds` definition and the mentioned alias
for all Nordic Semiconductor Development Kits that have a PWM node
with "okay" status.

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