ASoC: codecs: add SoundWire mockup device support

To test the host controller and bus management code, it is currently
required to have a physical SoundWire peripheral attached to the
bus. To help with pre-silicon or early hardware enablement, it would
be very useful to have a SoundWire 'mockup' device that is exposed in
platform firmware but does not drive any signal on the bus.

This is different to the existing ASoC 'dummy' codec uses for I2S/TDM,
the SoundWire spec makes it clear that a device that is not attached
to the bus is not permitted to interact with the bus, be it for
command/control or data.

This patch exposes a 'mockup' device, with a minimalist driver, with 4
partID values reserved by Intel for such test configurations. The
mockup device exposes one full-duplex DAI based on 2 ports (DP1 for
playback and DP8 for capture). The capture data port is just virtual,
such a mockup device is prevented by the SoundWire specification from
presenting any data generated by a Source port without being Attached.

All the callbacks exposed by the SoundWire Slave interface are
populated, even if they just return immediately. This is intentional
to describe what a minimal codec driver should do and implement and
help new codec vendors provide support for their devices.

Signed-off-by: Pierre-Louis Bossart <[email protected]>
Reviewed-by: Rander Wang <[email protected]>
Signed-off-by: Bard Liao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mark Brown <[email protected]>

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