rpi_pico2: rp2350: Add DMA support

Add initial support for the RP2350's DMA peripheral, allow tests
under drivers/dma/loop_transfer to run on on the Raspberry Pi Pico 2,
and update the board's documentation.

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