samples: fade_led: fix no led found error on rpi pico

Set the status of the pwm_leds node in the Raspberry Pi Pico
overlay to "okay". Without this change, the serial output shows
"PWM-based LED fade. Found 0 LEDs" and the led doesn't light up
on the Pico.

Signed-off-by: Saifan Rafiq <saifanr@gmail.com>