soc: qcom dt-bindings: Add APR bus bindings

This patch add dt bindings for Qualcomm APR (Asynchronous Packet Router)
bus driver. This bus is used for communicating with DSP which provides
audio and various other services to cpu.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Banajit Goswami <bgoswami@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>