docs: Add documentation for MHI bus

MHI (Modem Host Interface) is a communication protocol used by the
host processors to control and communicate with modems over a high
speed peripheral bus or shared memory. The MHI protocol has been
designed and developed by Qualcomm Innovation Center, Inc., for use
in their modems. This commit adds the documentation for the bus and
the implementation in Linux kernel.

This is based on the patch submitted by Sujeev Dias:
https://lkml.org/lkml/2018/7/9/987

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
[mani: converted to .rst and splitted the patch]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Jeffrey Hugo <jhugo@codeaurora.org>
Link: https://lore.kernel.org/r/20200220095854.4804-2-manivannan.sadhasivam@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>