boards: raspberrypi: rpi_pico2: include LED only for Pico 2

At the moment, "blinky" sample can be built for Pico 2 W, but it doesn't
work as expected because the board has no LED connected to GPIO.

rpi_pico2.dtsi is a common dtsi, used by Pico 2 and Pico 2 W, it
shouldn't contain board specific stuff (like LEDs).
So, include "rpi_pico-led.dtsi" directly in Pico 2 board dts.

Signed-off-by: Andrei Lalaev <andrey.lalaev@gmail.com>