dt-bindings: dmaengine: xilinx_dma: drop include-sg property

This property is not needed anymore, because the driver now autodetects it.
Delete references in documentation.

Cc: Rob Herring <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: [e-mail]
Cc: Radhey Shyam Pandey <[e-mail]>
Signed-off-by: Andrea Merello <[e-mail]>
Reviewed-by: Radhey Shyam Pandey <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Vinod Koul <[e-mail]>

Documentation/devicetree/bindings/dma/xilinx/xilinx_dma.txt

···
 Required properties for VDMA:
 - xlnx,num-fstores: Should be the number of framebuffers as configured in h/w.
 
-Optional properties:
-- xlnx,include-sg: Tells configured for Scatter-mode in
-	the hardware.
 Optional properties for AXI DMA:
 - xlnx,sg-length-width: Should be set to the width in bits of the length
 	register as configured in h/w. Takes values {8...26}. If the property