ASoC: zx-tdm: add bindings doc for zte's tdm controller

This patch adds dt-binding documentation for zte's tdm controller.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Reviewed-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>