QE: Add uqe_serial document to bindings

* Serial

Required Properties:

compatible : must be "fsl,<chip>-ucc-uart". For t1040, must be

"fsl,t1040-ucc-uart".

port-number : port number of UCC-UART

tx/rx-clock-name : should be "brg1"-"brg16" for internal clock source,

		   should be "clk1"-"clk28" for external clock source.

Example:

	ucc_serial: ucc@2200 {

		compatible = "fsl,t1040-ucc-uart";

		port-number = <0>;

		rx-clock-name = "brg2";

		tx-clock-name = "brg2";

	};