drivers: dma: rpi_pico: Refactor DMA files for Raspberry Pi SoCs

Avoid referring to Pico 2 (the name of a board). In this context,
RPI_PICO is used to refer to the (Zephyr) `SOC_FAMILY` rather than the
Pico 1 board. This clarifies common numerical values between the RP2040
and RP2350 SoC series, and enables existing DTS files to be used with
RP2350-based boards with fewer changes.

Remove the use of Zehpyr's `CONFIG_` macros from the device tree files,
and replace them with `SOC_SERIES`-specific files. Update the driver
implementation to conditionally include the correct file. Update
documentation and samples to match.

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>