boards: Raspberry Pi pico pwm led adjustment

The Raspberry Pi pico defines PWM leds, but on the Raspberry Pi pico
w-variant the gpio to the led is routed to the WiFi/Bluetooth module,
thus the led is not available.

Introduce a HAS_DT_PWM_LED define which allows devicetree overlays for
the rpi_pico board to distinguish between board variants with a pwm
controlled led, and those without.

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