Optimize flash writes on SAMD21

[mateusznowakdev]

In its current implementation, the machine.Flash.WriteAt() function on SAMD21 will copy data to the internal flash memory in 4-byte chunks -- even if the page buffer is actually 64 bytes long for all chip variants.

The existing code can be improved to get ~12x faster write speeds, which will be useful when #4844 (USB mass storage) is merged and SAMD21 support is added.

I get these results when writing 4kB of data:

	at 48MHz	at 8MHz
before	229ms	261ms
after	17ms	23ms

This will also make sure that no more than 4 writes are performed to the same flash row (256 bytes long). The datasheet says:

on this flash technology, a max number of 8 consecutive write is allowed per row. Once this number is reached, a row erase is mandatory.

Based on my long-term testing (working on a device that reads and writes a lot), it seems to work as expected. Data is saved to the expected memory location and can be read after the reset. It would be nice if someone else could take a look.

Of course, this is a breaking change. If some application assumes the block size of 4, it will not work properly.

There's also SAMD51 that could benefit from similar change, but I don't have any device to test with.

References:

• Datasheet: https://ww1.microchip.com/downloads/en/DeviceDoc/SAM_D21_DA1_Family_DataSheet_DS40001882F.pdf
  • "Table 10-3. SAM D21 Flash Memory Parameters"
  • "Table 37-42. Maximum Operating Frequency" and the notice below the table
  • "22.6.4.3 NVM Write"

[deadprogram]

Thank you very much for this improvement @mateusznowakdev

Of course, this is a breaking change. If some application assumes the block size of 4, it will not work properly.

External consumers should be using the WriteBlockSize() so should not impact any well-behaved code.

I agree that similar change for SAMD51 would benefit. I can test it if you wanted to make this change in another PR.

In any case, thanks again for working on this. Now merging.