ASoC: samsung,tm2-audio DT binding documentation update

This patch documents additional entries of the audio-codec and
i2s-controller properties required for the HDMI audio support.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>