Set coherent_dma_mask to DMA_BIT_MASK(32) for dma40 platform_device, as
without this DMA allocations were failing with the error:

dma40 dma40.0: coherent DMA mask is unset

when booting without device-tree.
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>