rpmsg: glink: Add GLINK signal support for RPMSG

Add support to handle SMD signals to RPMSG over GLINK. SMD signals
mimic serial protocol signals to notify of ports opening and closing.
This change affects the glink_pkt, rpmsg core and glink drivers.

Change-Id: I0b09219c793d23c65bae8ca747fc480ae6db47a6
Signed-off-by: Chris Lew <clew@codeaurora.org>