dt-bindings: cpufreq: add virtual cpufreq device

Adding bindings to represent a virtual cpufreq device.

Virtual machines may expose MMIO regions for a virtual cpufreq device
for guests to read performance information or to request performance
selection. The virtual cpufreq device has an individual controller for
each performance domain. Performance points for a given domain can be
normalized across all domains for ease of allowing for virtual machines
to migrate between hosts.

Co-developed-by: Saravana Kannan <[e-mail]>
Signed-off-by: Saravana Kannan <[e-mail]>
Signed-off-by: David Dai <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Signed-off-by: Viresh Kumar <[e-mail]>

Documentation/devicetree/bindings/cpufreq/qemu,virtual-cpufreq.yaml

···
+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/cpufreq/qemu,virtual-cpufreq.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Virtual CPUFreq
+
+maintainers:
+  - David Dai <[e-mail]>
+  - Saravana Kannan <[e-mail]>
+
+description:
+  Virtual CPUFreq is a virtualized driver in guest kernels that sends performance
+  selection of its vCPUs as a hint to the host through MMIO regions. Each vCPU
+  is associated with a performance domain which can be shared with other vCPUs.
+  Each performance domain has its own set of registers for performance controls.
+
+properties:
+  compatible:
+    const: qemu,virtual-cpufreq
+
+  reg:
+    maxItems: 1
+    description:
+      Address and size of region containing performance controls for each of the
+      performance domains. Regions for each performance domain is placed
+      contiguously and contain registers for controlling DVFS(Dynamic Frequency
+      and Voltage) characteristics. The size of the region is proportional to
+      total number of performance domains.
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    soc {
+      #address-cells = <1>;
+      #size-cells = <1>;
+
+      cpufreq@1040000 {
+        compatible = "qemu,virtual-cpufreq";
+        reg = <0x1040000 0x2000>;
+      };
+    };