powerpc/8xx: CONFIG_DEBUG_PAGEALLOC requires ITLBmiss for kernel addresses

When CONFIG_DEBUG_PAGEALLOC is activated, the initial TLB mapping gets
flushed to track accesses to wrong areas. Therefore, kernel addresses
will also generate ITLB misses.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>