remoteproc: imx_rproc: Request mbox channel later

It is possible that when remote processor crash, the communication
channel will be broken with garbage value in mailbox, such as
when Linux is issuing a message through mailbox, remote processor
crashes, we need free & rebuild the mailbox channels to make sure
no garbage value in mailbox channels.

So move the request/free to start/stop for managing remote procesosr in
Linux, move to attach/detach for remote processor is out of control of
Linux.

Previous, we just request mbox when attach for CM4 boot early before
Linux, but if mbox defer probe, remoteproc core will do resource cleanup
and corrupt resource table for later probe.

So move request mbox ealier and still keep mbox request when attach
for self recovery case, but keep a check when request/free mbox.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20221021041526.3696483-7-peng.fan@oss.nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>