axi_dma: Generate parameter for SG

Directly specify if SG is included or not.
For example Vivado loopback hw design has no stream connected
but SG is included.
This patch doesn't break backward compatibility just add
one more parameter which hasn't be exported to DTS.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>