ASoC: ak4613: add TDM256 test support

Merge series from Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>:

Renesas is the only user of ak4613 driver on upstream for now.
It has STEREO/TDM512/TDM256/TDM128 mode, but STEREO only is used,
because of Renesas board connection.

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

base-commit: 3123109284176b1532874591f7c81f3837bbdc17