FROMLIST: rpmsg: core: Add signal API support

RPMSG unifies various transports that provide IPC to a remote proc.
Some of these transports require some set of side band signalling in
order to meet the specifications of the protocol they implement.

The GLINK native transport supports the tty serial signals to start
communication with modems that expect to receive the DTR serial signal.

Extend the rpmsg core with an interface to send and receive sideband
signals for the transports that need it.

Bug: 161128971
Link: https://lore.kernel.org/lkml/1593182819-30747-2-git-send-email-deesin@codeaurora.org/


Change-Id: I54539d8ddce1bfaec9016c2bec9b5a1372601995
Signed-off-by: Chris Lew <clew@codeaurora.org>