rpi_pico: Fix DTC warnings concerning the flash

The addresses of the flash and flash controller of the RP2040
SoC were mixed up. There was no clear distinction between the
flash and the flash controller, which was unclear but also
caused a DTC warning.
This commit makes the distinction clearer: The SSI peripheral at
0x18000000 is the flash controller, and the flash itself starts
at 0x10000000. The flash driver and rpi_pico.dts were fixed
accordingly.

Signed-off-by: Yonatan Schachter <yonatan.schachter@gmail.com>