microblaze: Fix pfn_valid() for noMMU

Configuring DEBUG_SLAB causes a noMMU kernel to die during initialization
with an invalid virtual address panic in kfree_debugcheck().
The panic is due to an improper definition of pfn_valid().

Signed-off-by: Steven J. Magnani <steve@digidescorp.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>