boards: nordic: nrf54l20pdk: Define PWM LED

Define PWM LED at nRF54L20pdk board definition.

Set status=okay for pwm20 node.
Add pincontrol for pwm20:OUT0 at P1.07 (LED1).
Add pwm_led1 node.
Add alias pwm-led0.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>