MLK-13994-1: ARM: imx7ulp: add acknowledgement for vlls message notification

MLK-13994-1: ARM: imx7ulp: add acknowledgement for vlls message notification

Now, M4 rpmsg add free buffer feature that free the reciever buffer and update
some rpmsg structure data in share memory(DDR) once M4 receive the message from
A7 side, that means M4 will access DDR after it receive the vlls message(A7 enter
VLLS mode quickly), thus M4 hang because DDR in slef-refresh...
This patch move the vlls message notification ahead to driver suspend and add
acknowledgement to make sure no any DDR access comes from M4 side after A7 enter
suspend.

Signed-off-by: Robin Gong <yibin.gong@nxp.com>