Binding for Silicon Labs Si5351a/b/c programmable i2c clock generator.

Reference
[1] Si5351A/B/C Data Sheet
    http://www.silabs.com/Support%20Documents/TechnicalDocs/Si5351.pdf

The Si5351a/b/c are programmable i2c clock generators with up to 8 output
clocks. Si5351a also has a reduced pin-count package (MSOP10) where only
3 output clocks are accessible. The internal structure of the clock
generators can be found in [1].

==I2C device node==

Required properties:
- compatible: shall be one of "silabs,si5351{a,a-msop,b,c}".
- reg: i2c device address, shall be 0x60 or 0x61.
- #clock-cells: from common clock binding; shall be set to 1.
- clocks: from common clock binding; list of parent clock
  handles, shall be xtal reference clock or xtal and clkin for
  si5351c only. Corresponding clock input names are "xtal" and
  "clkin" respectively.
- #address-cells: shall be set to 1.
- #size-cells: shall be set to 0.

Optional properties:
- silabs,pll-source: pair of (number, source) for each pll. Allows
  to overwrite clock source of pll A (number=0) or B (number=1).

==Child nodes==

Each of the clock outputs can be overwritten individually by
using a child node to the I2C device node. If a child node for a clock
output is not set, the eeprom configuration is not overwritten.

Required child node properties:
- reg: number of clock output.

Optional child node properties:
- silabs,clock-source: source clock of the output divider stage N, shall be
  0 = multisynth N
  1 = multisynth 0 for output clocks 0-3, else multisynth4
  2 = xtal
  3 = clkin (si5351c only)
- silabs,drive-strength: output drive strength in mA, shall be one of {2,4,6,8}.
- silabs,multisynth-source: source pll A(0) or B(1) of corresponding multisynth
  divider.
- silabs,pll-master: boolean, multisynth can change pll frequency.
- silabs,disable-state : clock output disable state, shall be
  0 = clock output is driven LOW when disabled
  1 = clock output is driven HIGH when disabled
  2 = clock output is FLOATING (HIGH-Z) when disabled
  3 = clock output is NEVER disabled

==Example==

/* 25MHz reference crystal */
ref25: ref25M {
	compatible = "fixed-clock";
	#clock-cells = <0>;
	clock-frequency = <25000000>;
};

i2c-master-node {

	/* Si5351a msop10 i2c clock generator */
	si5351a: clock-generator@60 {
		compatible = "silabs,si5351a-msop";
		reg = <0x60>;
		#address-cells = <1>;
		#size-cells = <0>;
		#clock-cells = <1>;

		/* connect xtal input to 25MHz reference */
		clocks = <&ref25>;
		clock-names = "xtal";

		/* connect xtal input as source of pll0 and pll1 */
		silabs,pll-source = <0 0>, <1 0>;

		/*
		 * overwrite clkout0 configuration with:
		 * - 8mA output drive strength
		 * - pll0 as clock source of multisynth0
		 * - multisynth0 as clock source of output divider
		 * - multisynth0 can change pll0
		 * - set initial clock frequency of 74.25MHz
		 */
		clkout0 {
			reg = <0>;
			silabs,drive-strength = <8>;
			silabs,multisynth-source = <0>;
			silabs,clock-source = <0>;
			silabs,pll-master;
			clock-frequency = <74250000>;
		};

		/*
		 * overwrite clkout1 configuration with:
		 * - 4mA output drive strength
		 * - pll1 as clock source of multisynth1
		 * - multisynth1 as clock source of output divider
		 * - multisynth1 can change pll1
		 */
		clkout1 {
			reg = <1>;
			silabs,drive-strength = <4>;
			silabs,multisynth-source = <1>;
			silabs,clock-source = <0>;
			pll-master;
		};

		/*
		 * overwrite clkout2 configuration with:
		 * - xtal as clock source of output divider
		 */
		clkout2 {
			reg = <2>;
			silabs,clock-source = <2>;
		};
	};
};