ASoC: qcom: Document LPASS CPU bindings

Add documentation to the sound directory of the
device-tree bindings for the QTi LPASS CPU DAI
device.

Signed-off-by: Kenneth Westfield <kwestfie@codeaurora.org>
Acked-by: Banajit Goswami <bgoswami@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>