microblaze: Use generic pci_mmap_resource_range()

Commit f7195824

 ("PCI: Add pci_mmap_resource_range() and use it for
ARM64") added this generic function with the intent of using it
everywhere and ultimately killing the old arch-specific implementations.

Let's get on with that eradication...

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>