MLK-11229 rpmsg: imx: enhance the rpmsg tty capability

- remove the max limitation of the string echo test.
- change the echo-ed string printk to be debug level,
take precautions against that the echo string sent by
m4 doesn't contained the end operator.
- Sometimes, string message transmission is divided by
1 byte size message, and sent to M4 core.
The root cause of this phenomena is that the test method is
not properly. The correct one is that the mxc_mcc_test.out
should be used to listen the tty message read back after
the echo command is issued. For example:
  - step 1, lanunch the mxc_mcc_tty_test.out in backend:
  ./mxc_mcc_tty_test.out /dev/ttyRPMSG 115200 R 512 512 &
  - Step 2, issue the echo command:
  echo <string> > /dev/ttyRPMSG

Signed-off-by: Richard Zhu <Richard.Zhu@freescale.com>