bcm2835-dma: Add bcm2835-dma: Add DMA_WIDE_SOURCE and DMA_WIDE_DEST flags

Use (reserved) bits 24 and 25 of the dreq value
(the second cell of the DT DMA descriptor) to request
that wide source reads or wide dest writes are required

Signed-off-by: Dom Cobley <popcornmix@gmail.com>