dt-bindings: devfreq: Add DT bindings for devfreq_simple device

Devfreq simple device

devfreq-simple-dev is a device that represents a simple device that cannot do

any status reporting and uses a clock that can be scaled by one of more

devfreq governors.  It provides a list of usable frequencies for the device

and some additional optional parameters.

Required properties:

- compatible:		Must be "devfreq-simple-dev"

- clock-names:		Must be "devfreq_clk"

- clocks:		Must refer to the clock that's fed to the device.

- freq-tbl-khz:		A list of usable frequencies (in KHz) for the device

			clock.

Optional properties:

- polling-ms:	Polling interval for the device in milliseconds. Default: 50

- governor:	Initial governor to user for the device. Default: "performance"

Example:

	qcom,cache {

		compatible = "devfreq-simple-dev";

		clock-names = "devfreq_clk";

		clocks = <&clock_krait clk_l2_clk>;

		polling-ms = 50;

		governor = "cpufreq";

		freq-tbl-khz =

			<  300000 >,

			<  345600 >,

			<  422400 >,

			<  499200 >,

			<  576000 >,

			<  652800 >,

			<  729600 >,

			<  806400 >,

			<  883200 >,

			<  960000 >,

			< 1036800 >,

			< 1113600 >,

			< 1190400 >,

			< 1267200 >,

			< 1344000 >,

			< 1420800 >,

			< 1497600 >,

			< 1574400 >,

			< 1651200 >,

			< 1728000 >;

	};