dt-bindings: cpufreq: add bindings for MediaTek cpufreq HW

Add devicetree bindings for MediaTek HW driver.
Signed-off-by: Hector.Yuan <hector.yuan@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

Documentation/devicetree/bindings/cpufreq/cpufreq-mediatek-hw.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/cpufreq/cpufreq-mediatek-hw.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MediaTek's CPUFREQ Bindings
+
+maintainers:
+  - Hector Yuan <hector.yuan@mediatek.com>
+
+description:
+  CPUFREQ HW is a hardware engine used by MediaTek SoCs to
+  manage frequency in hardware. It is capable of controlling
+  frequency for multiple clusters.
+
+properties:
+  compatible:
+    const: mediatek,cpufreq-hw
+
+  reg:
+    minItems: 1
+    maxItems: 2
+    description:
+      Addresses and sizes for the memory of the HW bases in
+      each frequency domain. Each entry corresponds to
+      a register bank for each frequency domain present.
+
+  "#performance-domain-cells":
+    description:
+      Number of cells in a performance domain specifier.
+      Set const to 1 here for nodes providing multiple
+      performance domains.
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - "#performance-domain-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    cpus {
+            #address-cells = <1>;
+            #size-cells = <0>;
+
+            cpu0: cpu@0 {
+                device_type = "cpu";
+                compatible = "arm,cortex-a55";
+                enable-method = "psci";
+                performance-domains = <&performance 0>;
+                reg = <0x000>;
+            };
+    };
+
+    /* ... */
+
+    soc {
+        #address-cells = <2>;
+        #size-cells = <2>;
+
+        performance: performance-controller@11bc00 {
+            compatible = "mediatek,cpufreq-hw";
+            reg = <0 0x0011bc10 0 0x120>, <0 0x0011bd30 0 0x120>;
+
+            #performance-domain-cells = <1>;
+        };
+    };