rpmsg: virtio_rpmsg: set rpmsg_buf_size customizable

Rpmsg buffer size is currently fixed to 512 bytes.
This patch introduces a new capability in struct virtproc_info
to tune shared buffer size between host and coprocessor
according to the needs.

Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>