rmnet_ctl: Rmnet control driver

Added RmNet control driver to communicate with dedicated MHI channels.
The driver provides interfaces for clients to register QMAP command
handlers and transmit QMAP packets.

Change-Id: Ib70ce1e2e6e5c6248888a515bfc91e09e042c534
Acked-by: Weiyi Chen <weiyic@qti.qualcomm.com>
Signed-off-by: Conner Huff <chuff@codeaurora.org>