serial: add device tree binding documentation for Conexant USART

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/digicolor-usart.txt

+Binding for Conexant Digicolor USART
+
+Note: this binding is only applicable for using the USART peripheral as
+UART. USART also support synchronous serial protocols like SPI and I2S. Use
+the binding that matches the wiring of your system.
+
+Required properties:
+- compatible : should be "cnxt,cx92755-usart".
+- reg: Should contain USART controller registers location and length.
+- interrupts: Should contain a single USART controller interrupt.
+- clocks: Must contain phandles to the USART clock
+  See ../clocks/clock-bindings.txt for details.
+
+Note: Each UART port should have an alias correctly numbered
+in "aliases" node.
+
+Example:
+	aliases {
+		serial0 = &uart0;
+	};
+
+	uart0: uart@f0000740 {
+		compatible = "cnxt,cx92755-usart";
+		reg = <0xf0000740 0x20>;
+		clocks = <&main_clk>;
+		interrupts = <44>;
+	};