Binding for simple divider clock.

This binding uses the common clock binding[1].  It assumes a
register-mapped adjustable clock rate divider that does not gate and has
only one input clock or parent.  By default the value programmed into
the register is one less than the actual divisor value.  E.g:

register value		actual divisor value
0			1
1			2
2			3

This assumption may be modified by the following optional properties:

index-starts-at-one - valid divisor values start at 1, not the default
of 0.  E.g:
register value		actual divisor value
1			1
2			2
3			3

index-power-of-two - valid divisor values are powers of two.  E.g:
register value		actual divisor value
0			1
1			2
2			4

index-allow-zero - same as index_one, but zero is divide-by-1.  E.g:
register value		actual divisor value
0			1
1			1
2			2

Additionally a table of valid dividers may be supplied like so:

	table = <4 0>, <8, 1>;

where the first value in the pair is the divider and the second value is
the programmed register bitfield.

The binding must also provide the register to control the divider and
the mask for the corresponding control bits.  Optionally the number of
bits to shift that mask, if necessary.  If the shift value is missing it
is the same as supplying a zero shift.

[1] Documentation/devicetree/bindings/clock/clock-bindings.txt

Required properties:
- compatible : shall be "divider-clock".
- #clock-cells : from common clock binding; shall be set to 0.
- clocks : link to phandle of parent clock
- reg : base address for register controlling adjustable divider
- bit-mask : arbitrary bitmask for programming the adjustable divider

Optional properties:
- clock-output-names : from common clock binding.
- table : array of integer pairs defining divisors & bitfield values
- bit-shift : number of bits to shift the bit-mask, defaults to
  (ffs(mask) - 1) if not present
- minimum-divider : min divisor for dividing the input clock rate, only
  needed if the first divisor is offset from the default value
- maximum-divider : max divisor for dividing the input clock rate, only
  needed if the max divisor is less than (mask + 1).
- index-starts-at-one : valid divisor programming starts at 1, not zero
- index-power-of-two : valid divisor programming must be a power of two
- index-allow-zero : implies index-one, and programming zero results in
  divide-by-one
- hiword-mask : lower half of the register programs the divider, upper
  half of the register indicates bits that were updated in the lower
  half

Examples:
	clock_foo: clock_foo at 4a008100 {
		compatible = "divider-clock";
		#clock-cells = <0>;
		clocks = <&clock_baz>;
		reg = <0x4a008100 0x4>
		mask = <0x3>
		maximum-divider = <3>
	};

	clock_bar: clock_bar at 4a008108 {
		#clock-cells = <0>;
		compatible = "divider-clock";
		clocks = <&clock_foo>;
		reg = <0x4a008108 0x4>;
		mask = <0x1>;
		shift = <0>;
		table = < 4 0 >, < 8 1 >;
	};