rpmsg: glink: Add support for TX intents

Intents are nothing but pre-allocated buffers of
appropriate size that are allocated on the local
side and communicated to the remote side and the
remote stores the list of intent ids that it is
informed.

Later when remote side is intenting to send data,
it picks up a right intent (based on the size) and
sends the data buffer and the intent id. Local side
receives the data and copies it to the local intent
buffer.

The whole idea is to avoid stalls on the transport
for allocating memory, used for copy based transports.

When the remote request to allocate buffers using
CMD_RX_INTENT_REQ, we allocate buffers of requested
size, store the buffer id locally and also communicate
the intent id to the remote.

Change-Id: I5b29536fa04e4852ee01b707118f82d441295958
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Sricharan R <sricharan@codeaurora.org>