DMA: Freescale: revise device tree binding document

This patch updates the discription of each type of DMA controller and its
channels, it is preparation for adding another new DMA controller binding, it
also fixes some defects of indent for text alignment at the same time.

Signed-off-by: Hongbo Zhang <hongbo.zhang@freescale.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>