QE: Add uqe_serial document to bindings

Add uqe_serial document to
Documentation/devicetree/bindings/powerpc/fsl/cpm_qe/uqe_serial.txt
Signed-off-by: NZhao Qiang <qiang.zhao@nxp.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NScott Wood <oss@buserror.net>

Documentation/devicetree/bindings/powerpc/fsl/cpm_qe/uqe_serial.txt

+* 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";
+	};