dt-bindings: serial: Add binding for uartlite

The uartlite devicetree binding was missed out.
Add the binding documentation for uartlite that is already in use.
Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/xlnx,opb-uartlite.txt

+Xilinx Axi Uartlite controller Device Tree Bindings
+---------------------------------------------------------
+
+Required properties:
+- compatible		: Can be either of
+				"xlnx,xps-uartlite-1.00.a"
+				"xlnx,opb-uartlite-1.00.b"
+- reg			: Physical base address and size of the Axi Uartlite
+			  registers map.
+- interrupts		: Should contain the UART controller interrupt.
+
+Optional properties:
+- port-number		: Set Uart port number
+- clock-names		: Should be "s_axi_aclk"
+- clocks		: Input clock specifier. Refer to common clock bindings.
+
+Example:
+serial@800c0000 {
+	compatible = "xlnx,xps-uartlite-1.00.a";
+	reg = <0x0 0x800c0000 0x10000>;
+	interrupts = <0x0 0x6e 0x1>;
+	port-number = <0>;
+};