rpi_pico2: rp2350: Add PIO support and extend samples

Add initial support for the RP2350's PIO peripherals, extend the
existing example under samples/boards/raspberrypi/rpi_pico/uart_pio to
demonstrate this on the Raspberry Pi Pico 2, and update the board's
documentation.

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>
Signed-off-by: Manuel Aebischer <manuel.aebischer@belden.com>