rapidio/tsi721: add PCIe MRRS override parameter

rapidio/tsi721: add PCIe MRRS override parameter

Add PCIe Maximum Read Request Size (MRRS) adjustment parameter to allow
users to override configuration register value set during PCIe bus
initialization.

Performance of Tsi721 device as PCIe bus master can be improved if MRRS
is set to its maximum value (4096 bytes).  Some platforms have
limitations for supported MRRS and therefore the default value should be
preserved, unless it is known that given platform supports full set of
MRRS values defined by PCI Express specification.

Link: http://lkml.kernel.org/r/1469125134-16523-6-git-send-email-alexandre.bounine@idt.com
Signed-off-by: Alexandre Bounine <alexandre.bounine@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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>