[POWERPC] Fix rmb to order cacheable vs. noncacheable

lwsync is explicitly defined not to have any effect on the ordering of
accesses to device memory, so it cannot be used for rmb(). sync appears
to be the only barrier which fits the bill.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>