rpmsg: Add glink-bgcom driver for rpmsg framework

Add glink-bgcom driver which relies on bgcom as a transport
layer for sending data between various sub systems. Add
documentation file for the glink-bgcom driver.

Change-Id: I5eee03e6347a661dcefd083cea738b5d35fca848
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>