of/device: Set bus DMA mask as appropriate

When an explicit DMA limit is described by firmware, we need to remember
it regardless of how drivers might subsequently update their devices'
masks. The new bus_dma_mask field does that.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>