tty: cadence: Document DT binding

Add binding documentation for the Cadence UART.
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Acked-by: Rob Herring <robh@kernel.org>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

Documentation/devicetree/bindings/serial/cdns,uart.txt

+Binding for Cadence UART Controller
+
+Required properties:
+- compatible : should be "cdns,uart-r1p8", or "xlnx,xuartps"
+- reg: Should contain UART controller registers location and length.
+- interrupts: Should contain UART controller interrupts.
+- clocks: Must contain phandles to the UART clocks
+  See ../clocks/clock-bindings.txt for details.
+- clock-names: Tuple to identify input clocks, must contain "uart_clk" and "pclk"
+  See ../clocks/clock-bindings.txt for details.
+
+
+Example:
+	uart@e0000000 {
+		compatible = "cdns,uart-r1p8";
+		clocks = <&clkc 23>, <&clkc 40>;
+		clock-names = "uart_clk", "pclk";
+		reg = <0xE0000000 0x1000>;
+		interrupts = <0 27 4>;
+	};