greybus: svc: implement interface mailbox event

Implement the new interface mailbox-event operation.

The event is sent by the SVC under certain conditions when an interface
updates its mailbox value. Specifically, this event will be used to
implement the new mode-switch functionality.

Upon reception the AP verifies that the interface is known and that the
mailbox has the expected MAILBOX_GREYBUS value. If so, the interface is
disabled before being re-enabled (re-enumerated).

Note that during mode-switch, the interface will typically already be in
a disabled state when the mailbox is written (with the ES3 bootrom being
the notable exception).

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>