Quad SPI

see also SPI.

QSPI code supports single, dual, quad lanes. also supports both SDR and DDR modes.

• https://bitbucket.org/casl/c-class/src/3fba75dfbd0c64815eb8ec6dc965666812c44bae/src/peripherals/qspi/?at=master
• https://gitlab.raptorengineering.com/kestrel-collaboration/kestrel-litex/litex-boards/-/blob/a9ed41b6068236de7ce5b5ad0e80353be22817ca/litex_boards/targets/versa_ecp5.py#L43
• https://gitlab.raptorengineering.com/kestrel-collaboration/kestrel-firmware/bare-metal-firmware/-/blob/master/main.c#L2345