microblaze: add a dummy pgprot_noncached

Some device drivers call this, so add a macro
that pretends to do this. Since there is no
MMU support, it won't actually result in an
uncached mapping, though.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Simek <monstr@monstr.eu>