rpmsg: glink: Move the common glink protocol implementation to glink_native.c

Move the common part of glink core protocol implementation to
glink_native.c that can be shared with the smem based glink
transport in the later patches.

Acked-by: Arun Kumar Neelakantam <aneela@codeaurora.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>