powerpc: Use pr_devel() in do_dcache_icache_coherency()

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
   2036     368       8    2412     96c arch/powerpc/mm/pgtable.o

size after:
   text    data     bss     dec     hex filename
   1677     248       8    1933     78d arch/powerpc/mm/pgtable.o

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