Add a dummy SUPPLY widget connected to the ASP that forces the
chip registers to match the regmap cache when the ASP is
powered-up.

On a SoundWire system the ASP is free for use as a chip-to-chip
interconnect. This can be either for the firmware on multiple
CS35L56 to share reference audio; or as a bridge to another
device. If it is a firmware interconnect it is owned by the
firmware and the Linux driver should avoid writing the registers.
However. If it is a bridge then Linux may take over and handle
it as a normal codec-to-codec link.

CS35L56 is designed for SDCA and a generic SDCA driver would
know nothing about these chip-specific registers. So if the
ASP is being used on a SoundWire system the firmware sets up the
ASP registers. This means that we can't assume the default
state of the ASP registers. But we don't know the initial state
that the firmware set them to until after the firmware has been
downloaded and booted, which can take several seconds when
downloading multiple amps.

To avoid blocking probe() for several seconds waiting for the
firmware, the silicon defaults are assumed. This allows the machine
driver to setup the ASP configuration during probe() without being
blocked. If the ASP is hooked up and used, the SUPPLY widget
ensures that the chip registers match what was configured in the
regmap cache.

If the machine driver does not hook up the ASP, it is assumed that
it won't call any functions to configure the ASP DAI. Therefore
the regmap cache will be clean for these registers so a
regcache_sync() will not overwrite the chip registers. If the
DAI is not hooked up, the dummy SUPPLY widget will not be
invoked so it will never force-overwrite the chip registers.

Backport note:
This won't apply cleanly to kernels older than v6.6.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Fixes: e49611252900 ("ASoC: cs35l56: Add driver for Cirrus Logic CS35L56")
Link: https://msgid.link/r/20240129162737.497-8-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>