dt-bindings: dwc3-xilinx: Add missing comma in example

Trivial example fix.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/8fa5edcaa6b93859cfda97d080aad378e89c1b44.1611232967.git.michal.simek@xilinx.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/dwc3-xilinx.txt

@@ -19,7 +19,7 @@ Example device node:
 			#address-cells = <0x2>;
 			#size-cells = <0x1>;
 			compatible = "xlnx,zynqmp-dwc3";
-			clock-names = "bus_clk" "ref_clk";
+			clock-names = "bus_clk", "ref_clk";
 			clocks = <&clk125>, <&clk125>;
 			ranges;