QCOM Devfreq memory latency QoS voting device

Some Qualcomm Technologies, Inc. (QTI) chipsets have an interface to vote for
a memory latency QoS level. The qcom,devfreq-qoslat represents a device that
votes on this interface to request a particular memory latency QoS level.

Required properties:
- compatible:		Must be "qcom,devfreq-qoslat"
- operating-points-v2:	A phandle to the OPP v2 table that holds the supported
			QoS levels for the particular chipset. Currently, only
			OPP values of 0 and 1 are supported.
- mboxes:		A phandle to the mailbox used by this device to send
			requests to adjust the memory latency QoS level.
Optional properties:
- governor:		Initial governor to use for the device.
			Default: "powersave"

Example:

	qoslat_opp_table: qoslat-opp-table {
		compatible = "operating-points-v2";
		opp-0  {
			opp-hz = /bits/ 64 < 0 >;
		};

		opp-1 {
			opp-hz = /bits/ 64 < 1 >;
		};
	};

	qcom,devfreq-qoslat {
		compatible = "qcom,devfreq-qoslat";
		governor = "powersave";
		operating-points-v2 = <&qoslat_opp_table>;
		mboxes = <&qmp_aop 0>;
	};