samples: blinky_pwm: enable pwm_leds in rpi_pico overlay

Twister skips the blinky_pwm sample for the rpi_pico board because the
pwm-leds compatible is disabled.

Enable /pwm_leds node so that twister build the sample for the rpi_pico
board.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>