drivers: net: rmnet: Initial implementation

RmNet driver provides a transport agnostic MAP (multiplexing and
aggregation protocol) support in embedded and bridge modes. Module
provides virtual network devices which can be attached to any IP-mode
physical device. This will be used to provide all MAP functionality
on future hardware in a single consistent location.

CRs-Fixed: 2022292
Change-Id: I4dd0f4fcf00bbf9dcbec65cec82436d48a813ecc
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>