spi: microchip-core-qspi: Add regular transfers

Merge series from Conor Dooley <conor@kernel.org>:

This is a v2 of a patchset I sent about this time last year, adding the
regular transfer_one_message op to the microchip-core-qspi driver. In that
v1 Mark expressed his dislike for that op, so v2 is using
prepare/unprepare/transfer_one instead. The unprepare implementation still
contains the 750 us delay that the driver had back in v1. I've heard a
suggestion internally as to why this is needed, but it was unsubstantiated,
so I still have no justification for it. I held off on sending a v2 because
of a lack of explanation for the delay, but I don't wanna hold off forever
for something I might never understand.