mailbox: imx: Add context save/restore for suspend/resume

For "mem" mode suspend on i.MX8 SoCs, MU settings could be
lost because its power is off, so save/restore is needed
for MU settings during suspend/resume. However, the restore
can ONLY be done when MU settings are actually lost, for the
scenario of settings NOT lost in "freeze" mode suspend, since
there could be still IPC going on multiple CPUs, restoring the
MU settings could overwrite the TIE by mistake and cause system
freeze, so need to make sure ONLY restore the MU settings when
it is powered off, Anson fixes this by checking whether restore
is actually needed when resume.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>