ARM: dts: imx6sx: Remove unused property

Property 'dma-source' is not used anywhere, nor it is documented, so
let's just get rid of it.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6sx.dtsi

@@ -971,7 +971,6 @@ sai1: sai@021d4000 {
 				clock-names = "bus", "mclk1", "mclk2", "mclk3";
 				dma-names = "rx", "tx";
 				dmas = <&sdma 31 24 0>, <&sdma 32 24 0>;
-				dma-source = <&gpr 0 15 0 16>;
 				status = "disabled";
 			};
 
@@ -991,7 +990,6 @@ sai2: sai@021dc000 {
 				clock-names = "bus", "mclk1", "mclk2", "mclk3";
 				dma-names = "rx", "tx";
 				dmas = <&sdma 33 24 0>, <&sdma 34 24 0>;
-				dma-source = <&gpr 0 17 0 18>;
 				status = "disabled";
 			};