QCOM Devfreq firmware device

Some Qualcomm Technologies, Inc. (QTI) chipsets have a firmware that
offloads the steps for frequency switching. It provides a table of
supported frequencies and a register to request one of the supported
freqencies.

The qcom,devfreq-fw represents this firmware as a device. Sometimes,
multiple entities want to vote on the frequency request that is sent to the
firmware. The qcom,devfreq-fw-voter represents these voters as child
devices of the corresponding qcom,devfreq-fw device.

Required properties:
- compatible:		Must be "qcom,devfreq-fw" or "qcom,devfreq-fw-voter"
Only for qcom,devfreq-fw:
- reg:			Pairs of physical base addresses and region sizes of
			memory mapped registers.
- reg-names:		Names of the bases for the above registers.
			Required register regions are:
			- "en-base": address of register to check if the
			  firmware is enabled.
			- "ftbl-base": address region for the frequency
			  table.
			- "perf-base": address of register to request a
			  frequency.
			Optional register region:
			- "pstate-base": address of register to request
			  for current performance state (Mandatory if
			  qcom,support-panic-notifier property is added).
Optional properties:
- qcom,ftbl-row-size:    Size of the LUT row size.
- qcom,support-panic-notifier:	Support for panic notifier for l3-domain.

Example:

	qcom,devfreq-l3 {
		compatible = "qcom,devfreq-fw";
		reg-names = "en-base", "ftbl-base", "perf-base";
		reg = <0x18321000 0x4>, <0x18321110 0x600>, <0x18321920 0x4>;

		qcom,cpu0-l3 {
			compatible = "qcom,devfreq-fw-voter";
		};

		qcom,cpu4-l3 {
			compatible = "qcom,devfreq-fw-voter";
		};
	};