ASoC: ak4613: add TDM256 support

ASoC: ak4613: add TDM256 support

AK4613 has STEREO/TDM512/TDM256/TDM128 mode.
Renesas is the only user of ak4613 on upstream for now,
and is using it as STEREO mode, because of board connection.
Thus, current driver is supporting STEREO mode only, and other
modes are not supported.

But I noticed that I can try first 2ch out of TDM256 mode 8ch
Playback even in such a situation.

But because of board connection, I can't test full TDM256 mode,
and/or other TDM mode. Thus I don't want to add new DT propaty for now.
This patch enables TDM256 mode test by "ifdef style",
but it has no effect to current supported STEREO mode.
You can define AK4613_ENABLE_TDM_TEST to try TDM256 mode.

Please don't hesitate to break current code if you can add full TDM256
and/or other TDM mode. You don't need to care compatibility with Renesas.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/878rskp9l0.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>