ASoC: dt-bindings: rt1015p: add document

Adds DT binding document for rt1015p.

Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20200910042949.3795682-3-tzungbi@google.com


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