dt-bindings: imx-cpufreq-dt: Document opp-supported-hw usage

i.MX CPUFreq-DT OPP bindings

================================

Certain i.MX SoCs support different OPPs depending on the "market segment" and

"speed grading" value which are written in fuses. These bits are combined with

the opp-supported-hw values for each OPP to check if the OPP is allowed.

Required properties:

--------------------

For each opp entry in 'operating-points-v2' table:

- opp-supported-hw: Two bitmaps indicating:

  - Supported speed grade mask

  - Supported market segment mask

    0: Consumer

    1: Extended Consumer

    2: Industrial

    3: Automotive

Example:

--------

opp_table {

	compatible = "operating-points-v2";

	opp-1000000000 {

		opp-hz = /bits/ 64 <1000000000>;

		/* grade >= 0, consumer only */

		opp-supported-hw = <0xf>, <0x3>;

	};

	opp-1300000000 {

		opp-hz = /bits/ 64 <1300000000>;

		opp-microvolt = <1000000>;

		/* grade >= 1, all segments */

		opp-supported-hw = <0xe>, <0x7>;

	};

}