MLK-13981-2 rpmsg: imx: enable rpmsg virt tty

MLK-13981-2 rpmsg: imx: enable rpmsg virt tty

Message can be transferred between remote
device and iMX7ULP M4.
Then the message can be transferred between A7
and M4 by rpmsg channel.
demo howto:
- insmode the imx_rpmsg_tty.ko module after
login A7/Linux.
- Receive messages. Used the following command
to dump out the msg from the virtual tty.
./unit_tests/mxc_mcc_tty_test.out /dev/ttyRPMSG 115200 R 100 1000 &
- Send: use the following command to send the
message to M4.
echo <your string message> /dev/ttyRPMSG

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>