devicetree: serial: Add documentation for imx serial

Add documentation for imx serial.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>

Documentation/devicetree/bindings/serial/fsl-imx-uart.txt

+* Freescale i.MX UART controller
+
+Required properties:
+- compatible : should be "fsl,imx21-uart"
+- reg : Address and length of the register set for the device
+- interrupts : Should contain UART interrupt number
+
+Optional properties:
+- fsl,uart-has-rtscts: indicate that RTS/CTS signals are used
+
+Note: Each uart controller should have an alias correctly numbered
+in "aliases" node.
+
+Example:
+
+- From imx51.dtsi:
+aliases {
+	serial0 = &uart1;
+	serial1 = &uart2;
+	serial2 = &uart3;
+};
+
+uart1: serial@73fbc000 {
+	compatible = "fsl,imx51-uart", "fsl,imx21-uart";
+	reg = <0x73fbc000 0x4000>;
+	interrupts = <31>;
+	status = "disabled";
+}
+
+- From imx51-babbage.dts:
+uart1: serial@73fbc000 {
+	fsl,uart-has-rtscts;
+	status = "okay";
+};
+