microblaze: Provide pgprot_device/writecombine macros for nommu

These macros are required for generic mmap.c implementation created by:
commit f7195824 ("PCI: Add pci_mmap_resource_range() and use it for
ARM64") which Microblaze is going to use.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Suggested-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>