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.

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>