powerpc: convert CONFIG_PPC_MERGE to CONFIG_PPC for legacy io checks

Now that arch/ppc is dead CONFIG_PPC_MERGE is always defined for all
powerpc platforms and we want to get rid of CONFIG_PPC_MERGE use
CONFIG_PPC instead.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>