rpmsg: glink: Introduce glink smem based transport

The glink protocol supports different types of
transports (shared memory). With the core protocol
remaining the same, the way the transport's memory is
probed and accessed is different. So add support for
glink's smem based transports.

Add a new smem transport register function and the
fifo accessors for the same.

Change-Id: I2c7661dfe72070ac17e4202efaa28c258569575e
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Chris Lew <clew@codeaurora.org>