microblaze/PCI: fix "io_offset undeclared" error

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 58de74b8053]
Signed-off-by: Hiroo MATSUMOTO <matsumoto.hiroo@jp.fujitsu.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>