drivers: flash: rpi_pico: Modifications to support rp2350

The exisiting flash driver had some quite complicated implementations
for edge cases where partial pages where written. This commit modifies
the driver to always use the flash_range_program function from boot ROM.
This is a bit less efficient on partial page writes, but simplifies the
code a lot and adds compatibility to the new rp2350.

Signed-off-by: Manuel Aebischer <[email protected]>

Author: Manuel Aebischer