ASoC: samsung: Add DT bindings documentation for TM2 sound subsystem

This patch adds DT binding documentation for Exnos5433 based TM2
and TM2E boards sound subsystem.

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