powerpc: Cleanup & use pr_devel() in arch/powerpc/mm/slb.c

pr_debug() can now result in code being generated even when DEBUG
is not defined. That's not really desirable in some places.

With CONFIG_DYNAMIC_DEBUG=y:

size before:
   text    data     bss     dec     hex filename
   3261     416       4    3681     e61 arch/powerpc/mm/slb.o

size after:
   text    data     bss     dec     hex filename
   2861     248       4    3113     c29 arch/powerpc/mm/slb.o

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>