Add a basic blocking QSPI interface

[X-yl]

This module implements the QuadSPI interface which allows high speed communication with external flash memory.

Limitations:
- Interrupts are not supported.
- Status polling mode is not supported.

[X-yl]

Thanks for your suggestions, I've amended the commit to incorporate them. Let me know if any other changes are required :)

[X-yl]

Added #[cfg(feature = "quadspi")] to the RccBus stuff.

[burrbull]

Could you add example?

[burrbull]

Thanks for your suggestions, I've amended the commit to incorporate them. Let me know if any other changes are required :)

6650964

[X-yl]

Applied your change and added an example using a W25Q series flash chip.

[X-yl]

(Adjusted clock prescaler in example for max speed)

[burrbull]

Is Speed::VeryHigh required by QSPI / default?

[X-yl]

Is Speed::VeryHigh required by QSPI / default?

I don't think it's required, but that was the configuration generated by CubeMX so I used it as is. From my testing, VeryHigh is required for prescaler zero but it works without setting the speed for higher prescaler.

[burrbull]

I don't think it's required, but that was the configuration generated by CubeMX so I used it as is. From my testing, VeryHigh is required for prescaler zero but it works without setting the speed for higher prescaler.

516a009

[X-yl]

Fixed clippy suggestions

[burrbull]

bors r+

[bors]

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