bus: mhi: ep: Add support for creating and destroying MHI EP devices

This commit adds support for creating and destroying MHI endpoint devices.
The MHI endpoint devices binds to the MHI endpoint channels and are used
to transfer data between MHI host and endpoint device.

There is a single MHI EP device for each channel pair. The devices will be
created when the corresponding channels has been started by the host and
will be destroyed during MHI EP power down and reset.

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