microblaze: Fix IO space breakage after of_pci_range_to_resource() change

Commit 0b0b0893 "of/pci: Fix the conversion of IO ranges into IO
resources" changed the behaviour of of_pci_range_to_resource().

The issue is described here:
"powerpc/pci: Fix IO space breakage after of_pci_range_to_resource()
change"
(sha1: aeba3731)

Signed-off-by: Michal Simek <michal.simek@xilinx.com>