machine/rp2350: add flash support for rp2350

[mikesmitty]

Hi all, this still needs a bit of cleanup and I haven't had the chance to properly test it on hardware, but it builds cleanly. Figured I'd push it up to see if any big changes need to happen before I give it a final polish.

Fixes #4799

[mikesmitty]

Ok, I had a chance to test the console functions in https://github.com/tinygo-org/tinyfs/blob/release/examples/console/littlefs/machine/main.go and it seems to be working as far as I can tell, but I've not used tinyfs before

[eliasnaur]

I'm not the right reviewer for the details of the flash interface. Perhaps @deadprogram who authored the other implementation.

[mikesmitty]

Sounds good. It's mostly just copy/paste from pico-sdk with the RISC-V and RP2040-specific macro branches trimmed out and I grouped everything by source file with a header comment so it should hopefully be relatively straightforward to cross-reference. The only real changes I made were to mimic the conversion of flash_range_program()/flash_range_erase() to flash_range_write()/flash_erase_blocks() that was done in machine_rp2040_rom.go and to ditch the enum flash_devinfo_get_cs_size() used as a return value in favor of a uint8_t. It's only used once and only to check against the zero value from 4 register bits. I left that bit in even though it's only used for checking if a PSRAM chip exists to make it easier to add support for it down the road, but I can trim it out if that makes more sense