dma-mapping: add the missing ARCH_HAS_SYNC_DMA_FOR_CPU_ALL declaration

The patch adding the infrastructure failed to actually add the symbol
declaration, oops..

Fixes: faef8772 ("dma-noncoherent: add a arch_sync_dma_for_cpu_all hook")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Paul Burton <paul.burton@mips.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>