Support Pimoroni Plus 2 W flash size

[pelrun]

The Plus 2 W has 16MB of QSPI flash, but the Pico 2 build is hardcoded to 4MB.

This requires adjusting all of ATRANS0-3, which will also affect the flash_size() check in my PicoMite patch.

[pelrun]

Support for large partitions is in v2.3, but it requires building with a custom partitions.json.