soc: Added support for Raspberry Pi's RP2040 SoC

Added basic support for the RP2040 SoC. Support
includes booting and starting the kernel, on one
core only.

Signed-off-by: Yonatan Schachter <yonatan.schachter@gmail.com>
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>