powerpc/ps3: Use pr_devel() in ps3/mm.c

The non-debug case in ps3/mm.c uses pr_debug(), so that the compiler
still does type checks etc. and doesn't complain about unused
variables in the non-debug case.

However with DEBUG=n and CONFIG_DYNAMIC_DEBUG=y there's still code
generated for those pr_debugs().

size before:
   text    data     bss     dec     hex filename
  17553	   4112	     88	  21753	   54f9	arch/powerpc/platforms/ps3/mm.o

size after:
   text    data     bss     dec     hex filename
   7377	    776	     88	   8241	   2031	arch/powerpc/platforms/ps3/mm.o

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>