clk: Improve clock-indices binding documentation

  clocks by index. The names should reflect the clock output signal

  names for the device.

clock-indices:	   If the identifyng number for the clocks in the node

		   is not linear from zero, then the this mapping allows

		   the mapping of identifiers into the clock-output-names

		   array.

clock-indices:	   If the identifying number for the clocks in the node

		   is not linear from zero, then this allows the mapping of

		   identifiers into the clock-output-names array.

For example, if we have two clocks <&oscillator 1> and <&oscillator 3>:

		clock-output-names = "clka", "clkb";

	}

	This ensures we do not have any empty nodes in clock-output-names

	This ensures we do not have any empty strings in clock-output-names

==Clock consumers==