mailbox: apcs-ipc: Set suppress_bind_attrs

The apcs-ipc driver represents a hw block that will not be removed
dynamically. Additionally, the mailbox framework does not provide
any notification that a controller is shutting down to the mailbox
clients. Without cleanup these clients will crash.

Remove the ability to manually unbind the device via sysfs by setting
suppress_bind_attrs.

Change-Id: I3d4d8ea8b2c9193ce9782da7584bf51afdfe49e9
Signed-off-by: Chris Lew <clew@codeaurora.org>