ASoC: qcom: document apq8016 sbc machine driver bindings

This patch adds bindings for apq8016 sbc machine driver.
APQ8016 has 4 MI2S which can be configured to different sinks like
internal codec/external codec, this connection and various parameters
are controlled via 2 iomux registers.

Acked-by: Kenneth Westfield <kwestfie@codeaurora.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>