Mailbox: new mutext lock for h/w mailbox configuration

mailbox startup and shutdown are being executed against
a single H/W module, and a mailbox H/W module is totally
__independent__ of the registration of logical mailboxes.
So, an independent mutext should be used for startup and
shutdown.

Signed-off-by: Fernando Guzman Lugo <x0095840@ti.com>
Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>