powerpc: hash_page_sync should only be used on SMP & STD_MMU_32

Clean up the ifdefs so we only use hash_page_sync if we have
CONFIG_SMP && CONFIG_PPC_STD_MMU_32.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>