Add module parameter to allow load time configuration of available
RapidIO messaging mailboxes (MBOX1 - MBOX4).

Having a messaging MBOX selector mask allows to define which MBOXes are
controlled by the mport device driver and reserve some of them for
direct use by other drivers.

Link: http://lkml.kernel.org/r/1469125134-16523-7-git-send-email-alexandre.bounine@idt.comSigned-off-by: NAlexandre Bounine <alexandre.bounine@idt.com>
Tested-by: NBarry Wood <barry.wood@idt.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Andre van Herk <andre.van.herk@prodrive-technologies.com>
Cc: Barry Wood <barry.wood@idt.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>