ASoC: tas2770: Support setting the PDM TX slot

We don't actually support configuring the PDM input right now. Rather,
this is useful as a hack.

On Apple Silicon machines, amps are split between two I2S buses which
are logically ANDed internally at the SoC. Odd and even slot groups are
driven by amps on either bus respectively. Since the signals are ANDed,
unused slot groups must be driven as zero to avoid corrupting the data
from the other side.

On most recent machines (TAS2764-based), this is accomplished using the
"SDOUT zero mask" feature of that chip. Unfortunately, TAS2770 does not
support this. It does support zeroing out *all* unused slots, which
works well for machines with a single amp per I2S bus. That is all,
except one.

The 13" M1 MacBook Pro is the only machine using TAS2770 and two amps
per I2S bus:

L Bus: SPK0I SPK0V Hi-Z  Hi-Z  SPK2I SPK2V Hi-Z  Hi-Z
R Bus: Hi-Z  Hi-Z  SPK1I SPK2V Hi-Z  Hi-Z  SPK3I SPK3V

To ensure uncorrupted data, we need to force all the Hi-Z periods to
zero. We cannot use the "force all zero" feature, as that would cause a
bus conflict between both amps. We can use the pull-down feature, but
that leaves a few bits of garbage on the trailing edge of the speaker
data, since the pull-down is weak.

This is where the PDM transmit feature comes in. With PDM grounded and
disabled (the default state), the PDM slot is transmitted as all zeroes.
We can use that to force a zero 16-bit slot after the voltage data for
each speaker, cleaning it up. Then the pull-down ensures the line stays
low for the subsequent slot:

L Bus: SPK0I SPK0V PDM0  PulDn SPK2I SPK2V PDM0  PulDn
R Bus: PDM0  PulDn SPK1I SPK2V PDM0  PulDn SPK3I SPK3V

Yes, this is a horrible hack, but it beats adding dummy slots that would
be visible to the userspace capture side. There may be some other way to
fix the logical AND behavior on the MCA side... that would make this
unnecessary.

("How does Apple deal with this"? - they don't, macOS does not use
IVSENSE on TAS2770 machines even though it's physically wired up,
but we want to do so on Linux.)

Reviewed-by: Neal Gompa <neal@gompa.dev>
Signed-off-by: Hector Martin <marcan@marcan.st>
Signed-off-by: James Calligeros <jcalligeros99@gmail.com>
Link: https://patch.msgid.link/20250406-apple-codec-changes-v5-2-50a00ec850a3@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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