ASoC: soc-dai: define possible idle TDM slot modes

Some audio devices, such as certain Texas Instruments codecs,
include configurable bus keepers. We currently don't have
a standardised way to configure such hardware, and instead
rely on the hardware initialising setting itself up into a
sane state. There are situations where this is insufficient,
however, and some platforms require more concrete guarantees
as to the state of the bus, and being able to explicitly
configure bus keepers enables this.

For example, some Apple Silicon machines have an odd bus topology where
the SDOUT pins of all codecs are split across two data lines, which
are summed via an OR gate in front of the receiving port on the
SoC's I2S peripheral. Each line must transmit 0 while a codec
on the other line is actively transmitting data, or the SoC
will receive garbage data. To do this, one codec on each line
must be configured to transmit zeroes during the other line's
active TDM slots.

Thus, we define seven possible bus-keeping modes that a device can
be in: NONE (UB/as initialised), OFF (explicitly disabled), ZERO
(actively transmit a 0), PULLDOWN, HIZ (floating), PULLUP, and
DRIVE_HIGH.

These will be consumed by CODEC/CPU drivers via a common DAI
op, enabling the explicit configuration of bus keepers where
required.

Signed-off-by: James Calligeros <jcalligeros99@gmail.com>
Link: https://patch.msgid.link/20260301-tdm-idle-slots-v3-4-c6ac5351489a@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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