drivers: mailbox: rpmh: Suppress driver bind/unbind feature

RPMH mailbox driver do not support bind/unbind using sysfs.
Set suppress bind attribute to disallow this.

Change-Id: Id80bc7d7f21bdd32543d78edf608dcb2b987a70f
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>