powerpc: Fix !CONFIG_PPC_NEED_DMA_SYNC_OPS build warning

Change #define stubs of dma_sync ops to be empty static inlines
to avoid build warning.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>