MAINTAINERS: Add entry for MHI networking drivers under MHI bus

The host MHI net driver was not listed earlier. So let's add both host and
endpoint MHI net drivers under MHI bus.

Cc: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>