ARM CPU memory latency monitor device

arm-memlat-mon is a device that represents the use of the PMU in ARM cores
to measure the parameters for latency driven memory access patterns.

Required structure:
An instance of arm-memlat-mon must be described in two levels of device nodes.
The first level describes the controller while the second level describes the
monitors that the controller manages. At least one monitor is required per
controller.

[First Level Nodes]
Required properties:
- compatible:			Must be "qcom,arm-memlat-cpugrp"
- qcom,cpulist:			List of CPU phandles to be monitored in a
				cluster. Must be a superset of cpulists
				described in second level nodes.

[Second Level Nodes]
Required properties:
- compatible:			Must be "qcom,arm-memlat-mon" or
				"qcom,arm-compute-mon"
- qcom,target-dev:		The DT device that corresponds to this master
				port
- qcom,core-dev-table:		A mapping table of core frequency to a required
				bandwidth vote at the given core frequency.
- qcom,cachemiss-ev:		The cache miss event that this monitor is
				supposed to measure. Optional for compute only.
Optional properties:
- qcom,cpulist:			List of CPU phandles to be monitored in a
				cluster. Must be a subset of the cpulist
				described in first level node. Defaults to
				cpulist in first level node if not specified.
- qcom,inst-ev:			The instruction count event that this monitor is
				supposed to measure. Defaults to 0x08 if not
				specified.
- qcom,stall-cycle-ev:		The stall cycle count that this monitor is
				supposed to measure. Assumes 100% stall if not
				specified.
- qcom,ddr-type:		Optional property indicates ddr type which can support
				different frequencies for a given target.

Example:

#define DDR_TYPE_LPDDR3	5
#define DDR_TYPE_LPDDR4X	7

	qcom,arm-memlat-cpugrp {
		compatible = "qcom,arm-memlat-cpugrp";
		qcom,cpulist = <&CPU0 &CPU1>;

		qcom,arm-memlat-mon {
			compatible = "qcom,arm-memlat-mon";
			qcom,target-dev = <&memlat0>;
			qcom,cachemiss-ev = <0x2A>;
			qcom,inst-ev = <0x08>;
			qcom,stall-cycle-ev = <0xE7>;
			ddr3-map {
				qcom,ddr-type = <DDR_TYPE_LPDDR3>;
				qcom,core-dev-table =
				<  300000 1525 >,
				<  499200 3143 >,
				< 1881600 5859 >;
			};
			ddr4-map {
				qcom,ddr-type = <DDR_TYPE_LPDDR4X>;
				qcom,core-dev-table =
					<  300000 1525 >,
					<  499200 3143 >,
					< 1881600 5859 >;
			};
		};
	};