Documentation: dma: Add documentation for the APM X-Gene SoC DMA device DTS binding

This patch adds documentation for the APM X-Gene SoC DMA device DTS binding

Signed-off-by: Rameshwar Prasad Sahu <rsahu@apm.com>
Signed-off-by: Loc Ho <lho@apm.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>