ARM: Orion5x - Restore parts of io.h, with rework

Commit 4d5fc58d

 (ARM: remove bunch of
now unused mach/io.h files) removed the orion5x io.h. Unfortunately,
this is still needed for the definition of IO_SPACE_LIMIT which
overrides the default 64K. All Orion based systems have 1Mbyte of IO
space per PCI[e] bus, and try to request_resource() this size. Orion5x
has two such PCI buses.

It is likely that the original, removed version, was broken. This
version might be less broken. However, it has not been tested on
hardware with a PCI card, let alone hardware with a PCI card with IO
capabilities.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Olof Johansson <olof@lixom.net>