bus: mhi: ep: Add support for processing command and TRE rings

Since we now have all necessary infrastructure, let's add support for
processing the command and TRE rings in the MHI endpoint stack. As a part
of the TRE ring processing, the channel read functionality is also added
that allows the MHI endpoint device to read data from the host over any
available MHI channel.

During the TRE ring processing, client driver will also be notified about
the data availability.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>