bus: mhi: ep: Add support for processing channel rings

Add support for processing the channel rings from host. For the channel
ring associated with DL channel, the xfer callback will simply invoked.
For the case of UL channel, the ring elements will be read in a buffer
till the write pointer and later passed to the client driver using the
xfer callback.

The client drivers should provide the callbacks for both UL and DL
channels during registration.

Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>