[RFC PATCH] Virtio SPI Linux driver.

[Harald Mommer <Harald.Mommer@xxxxxxxxxxxxxxx>]

This is a virtio SPI Linux driver which is intended to be compliant with
the proposed virtio SPI draft specification V4.

The virtio SPI driver was smoke tested on qemu using OpenSynergy's
proprietary virtio SPI device doing a SPI backend simulation on top of
next-20231027 and on Linux 6.1 with target hardware providing a physical
SPI backend device.

For discussion with the author (Haixu Cui <quic_haixcui@xxxxxxxxxxx>) of
the work in progress virtio SPI draft specification and with other
people who may be interested in virtio SPI.