ARM64: zynqmp: Add description for LPDDMA channel usage

LPDDMA default allows only secured access.
inorder to enable these dma channels,
one should ensure that it allows non secure access.
This patch updates the same.
Reported-by: NSai Pavan Boddu <saipava@xilinx.com>
Signed-off-by: NKedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

arch/arm/dts/zynqmp.dtsi

@@ -421,7 +421,10 @@
 			power-domains = <&pd_gpu>;
 		};
 
-		/* ADMA */
+		/* LPDDMA default allows only secured access. inorder to enable
+		 * These dma channels, Users should ensure that these dma
+		 * Channels are allowed for non secure access.
+		 */
 		lpd_dma_chan1: dma@ffa80000 {
 			status = "disabled";
 			compatible = "xlnx,zynqmp-dma-1.0";