ARM64: zynqmp: Add backward compatible string for uart

Mainline kernel has no r1p12 compatible string that's why console stops
to work with the latest DTS files. Append generic compatible string.
Keep in your mind that using this generic compatible string not all uart
features will be available.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/dts/zynqmp.dtsi

@@ -775,7 +775,7 @@
 		};
 
 		uart0: serial@ff000000 {
-			compatible = "cdns,uart-r1p12";
+			compatible = "cdns,uart-r1p12", "xlnx,xuartps";
 			status = "disabled";
 			interrupt-parent = <&gic>;
 			interrupts = <0 21 4>;
@@ -785,7 +785,7 @@
 		};
 
 		uart1: serial@ff010000 {
-			compatible = "cdns,uart-r1p12";
+			compatible = "cdns,uart-r1p12", "xlnx,xuartps";
 			status = "disabled";
 			interrupt-parent = <&gic>;
 			interrupts = <0 22 4>;