ARM: move PCI i/o resource setup into common code

With consolidation of the PCI i/o mappings, the i/o space is being
set to start at a PCI bus addr of 0x0 and fixed to 64K per bus. In
this case the core ARM PCI setup code can setup the i/o resource.

Currently, the resource is only setup if the platform did not setup
an i/o resource.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>