rpmsg: glink: Associate indirections for pipe fifo accessor's

With the intention of reusing the glink core protocol commands
and code across both rpm and smem based transports, the only thing
different is way of accessing the shared-memory of the transport
(FIFO). So put the fifo accessor's of the transport's pipe (rx/tx)
behind indirections, so that the rest of the code can be shared.

For this, have a qcom_glink_pipe that can be used in the common code
containing the indirections and wrap it with glink_rpm_pipe that contains
the transport specific members.

Change-Id: I6d5043ffd9a99aa96f685e6a7ad4e70f7904e996
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>