There is a compile error for microblaze pci because io_offset is not
declared. This patch adds declaration of io_offset.

[bhelgaas: I introduced this problem with 58de74b8]
Signed-off-by: NHiroo MATSUMOTO <matsumoto.hiroo@jp.fujitsu.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>