PSRAM module ready for integration

[suarezvictor]

I successfuly integrated a somewhat old core from @xobs into a newer version of LiteX, it works well, so support for SPI PSRAM could be mainstreamed,

#https://github.com/suarezvictor/micropython/commit/d7d2203cab9c2909395bbb5a9cff3d5016fa81a3#diff-f689a3f83aaaa1076106b25044284fc4d4c04e2cd3e173ef6a9d9e74e6840348R79

EDIT: this may be a simpler development, presenting it as flash (no interleave support thoug) https://github.com/litex-hub/litex-boards/pull/476/commits/b1df2c0f85fbcd5d4880ae71c05ac39b0abc4291

Original thread #https://github.com/litex-hub/litespi/issues/25