remoteproc: imx_proc: enable virtio/mailbox

Use virtio/mailbox to build connection between Remote Proccessors
and Linux. Add work queue to handle incoming messages.

Reviewed-by: Richard Zhu <hongxing.zhu@nxp.com>
Reviewed-by: Mathieu Poirier <mathieu.poirer@linaro.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/1615029865-23312-11-git-send-email-peng.fan@oss.nxp.com
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>