dt-bindings: serial: Add Milbeaut serial driver description

Add DT bindings document for Milbeaut serial driver.
Signed-off-by: Sugaya Taichi <sugaya.taichi@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/milbeaut-uart.txt

+Socionext Milbeaut UART controller
+
+Required properties:
+- compatible: should be "socionext,milbeaut-usio-uart".
+- reg: offset and length of the register set for the device.
+- interrupts: two interrupts specifier.
+- interrupt-names: should be "rx", "tx".
+- clocks: phandle to the input clock.
+
+Optional properties:
+- auto-flow-control: flow control enable.
+
+Example:
+	usio1: usio_uart@1e700010 {
+		compatible = "socionext,milbeaut-usio-uart";
+		reg = <0x1e700010 0x10>;
+		interrupts = <0 141 0x4>, <0 149 0x4>;
+		interrupt-names = "rx", "tx";
+		clocks = <&clk 2>;
+		auto-flow-control;
+	};