linux-imx

rpmsg: imx: enable the tx_block mechanism in the flow

- Enable the tx_block mechanism to make sure that every transmission is
  complete when mailbox is used.
- Refine the data exchange in the rpmsg_rx_callback and some info
  messages and codes comments.

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>