msm: rmnet_smux: Add RMNET over SMUX

RMNET over SMUX will support IP Traffic over HSUART. This is used in
Fusion 4 target between Application processor and QSC modem.

Change-Id: I50a8acf96dc85d10e574768dc21db672b6615924
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>