dt-bindings: sound: gtm601: Add description

This allows to us to document the channel and sampling
rate requirements.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: David Heidelberg <david@ixit.cz>
Link: https://lore.kernel.org/r/b6e85fdfaa87d7684a120ccedc1e07d8fe87957f.1691606520.git.agx@sigxcpu.org


Signed-off-by: Mark Brown <broonie@kernel.org>