rpmsg: glink: Update signals interface

Update the serial signal interface for rpmsg and glink to match the
android common kernel abi.

Change ABI whitelist to use renamed APIs for rpmsg_get/set_signals().

Change-Id: If531efa835b0bfb8405a5dfd265025b5c2754a61
Signed-off-by: Chris Lew <clew@codeaurora.org>