bus: mhi: ep: Introduce transfer_{to/from}_host APIs

In preparation for adding DMA support, let's introduce separate APIs for
copying the MHI buffers between the host and endpoint. The MHI controller
drivers can use the DMA operations on these APIs if supported and use iATU
for rest of the read/write operations using {read_from/write_to}_host APIs.

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