rpmsg: glink: Add pipe reset functionality

Some remote procs do not initialize the pipe when they are started
and assume the data in the pipe descriptors is valid. Add an optional
handler to clear the pipe descriptors and call this during removal
of the glink device to ensure a clean restart of glink.

Change-Id: I62871cc5d6fd052e234843645299fdc87a71185f
Signed-off-by: Chris Lew <clew@codeaurora.org>