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>

arch/microblaze/include/asm/pgtable.h

@@ -33,6 +33,8 @@ extern int mem_init_done;
 #define PAGE_KERNEL		__pgprot(0) /* these mean nothing to non MMU */
 
 #define pgprot_noncached(x)	(x)
+#define pgprot_writecombine	pgprot_noncached
+#define pgprot_device		pgprot_noncached
 
 #define __swp_type(x)		(0)
 #define __swp_offset(x)		(0)