microblaze: Add _PAGE_FILE macros to pgtable.h

We need to define _PAGE_FILE macro and change pte
functions. Microblaze use the same MMU as PowerPC
that's why we define _PAGE_FILE in the same style.
This change fixed remap_file_pages01 LTP test.

Signed-off-by: Michal Simek <monstr@monstr.eu>