net: Add MHI Endpoint network driver

Add a network driver for the Modem Host Interface (MHI) Endpoint network
driver that provides network interfaces to Qualcomm modems such as
SDX55 over MHI Endpoint bus.

The driver currently supports only IP_SW0 MHI channel that can be used
to route IP traffic from the modem CPU to host machine.

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