dt-bindings: arm: Add Calxeda system registers json-schema binding

The Calxeda system registers are a collection of MMIO register
controlling several more general aspects of the SoC.
Beside for some power management tasks this node is also somewhat
abused as the container for the clock nodes.

Add a binding in DT schema format using json-schema.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/calxeda/hb-sregs.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/calxeda/hb-sregs.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Calxeda Highbank system registers
+
+description: |
+  The Calxeda Highbank system has a block of MMIO registers controlling
+  several generic system aspects. Those can be used to control some power
+  management, they also contain some gate and PLL clocks.
+
+maintainers:
+  - Andre Przywara <andre.przywara@arm.com>
+
+properties:
+  compatible:
+    const: calxeda,hb-sregs
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    type: object
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    sregs@fff3c000 {
+        compatible = "calxeda,hb-sregs";
+        reg = <0xfff3c000 0x1000>;
+
+        clocks {
+            #address-cells = <1>;
+            #size-cells = <0>;
+
+            osc: oscillator {
+                #clock-cells = <0>;
+                compatible = "fixed-clock";
+                clock-frequency = <33333000>;
+            };
+        };
+    };