Add W25Q64JV QSPI /4 Boot2 file

[maxgerhardt]

A simple addition. Expands only the "Generic" variant by a "W25Q64JV QSPI /4" Boot2 option.

I was very tempted to also just add

# Pimoroni
MakeBoard("pimoroni_pga2040", "Pimoroni", "PGA2040", "0x2e8a", "0x1008", 250, "PIMORONI_PGA2040", 8, "boot2_w25q64jv_4_padded_checksum")

but the pimoroni_pga2040 would have just been a copy of the generic variant folder -- felt weird.

Resolves #927 more cleanly by giving them the possibility to use their flash chip on Quad-SPI mode (clock div 4) as intended and e.g. used by CircuitPython.

[earlephilhower]

Thanks for digging into this new flash. I think I will do the makeboards addition you mentioned just to avoid having the same problem pop up again. Having a variant just for the flash type isn't unique to this board, IIRC, and the bits are free...