devicetree: serial: Add documentation for imx serial

* 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";

};