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.

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>