Binding for Xilinx Clocking Wizard IP Core

This binding uses the common clock binding[1]. Details about the devices can be
found in the product guide[2].

[1] Documentation/devicetree/bindings/clock/clock-bindings.txt
[2] Clocking Wizard Product Guide
https://www.xilinx.com/support/documentation/ip_documentation/clk_wiz/v5_1/pg065-clk-wiz.pdf

Required properties:
 - compatible: Must be 'xlnx,clocking-wizard'
 - reg: Base and size of the cores register space
 - clocks: Handle to input clock
 - clock-names: Tuple containing 'clk_in1' and 's_axi_aclk'
 - clock-output-names: Names for the output clocks

Optional properties:
 - speed-grade: Speed grade of the device (valid values are 1..3)

Example:
	clock-generator@40040000 {
		reg = <0x40040000 0x1000>;
		compatible = "xlnx,clocking-wizard";
		speed-grade = <1>;
		clock-names = "clk_in1", "s_axi_aclk";
		clocks = <&clkc 15>, <&clkc 15>;
		clock-output-names = "clk_out0", "clk_out1", "clk_out2",
				     "clk_out3", "clk_out4", "clk_out5",
				     "clk_out6", "clk_out7";
	};