Add support for using different modes for each phase of an xspi operation

[dne]

This adds Config.modes and Xspi.configure_modes methods but keeps the existing API (which sets the same mode for all phases).

[richardeoin]

Nice that this can be added without breaking backwards compatibility! I think configure_modes should have some documentation ("use this if you want different modes for each phase..") but otherwise looks good.

If you're able to share, it would be great to know if you have tested this on hardware and what the results were.

[dne]

Nice that this can be added without breaking backwards compatibility! I think configure_modes should have some documentation ("use this if you want different modes for each phase..") but otherwise looks good.

Thanks for reviewing! I've amended the PR to improve the docs, and also attempting to fix the octospi variant that failed in CI.

If you're able to share, it would be great to know if you have tested this on hardware and what the results were.

I've used this successfully with the 'H745 and Infineon S25FL127S QSPI Flash (in quad mode), using the 4QOR and 4QPP commands ("quad output read" and "quad page program" with 4-byte addresses).

[richardeoin]

Thanks for the updates