dt-bindings: soc: bcm: add missing dt-binding document for bcm23550-cdc

Broadcom BCM23550 has a Cluster Dormant Control block that keeps the CPU
in idle state. A command needs to be sent to this block to bring the CPU
into running state.

This has been in use in mainline Linux since ~2016, but was never
documented. Add a dt-binding document for it.
Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230508135930.GA27583@standask-GA-A55M-S2HPSigned-off-by: Florian Fainelli <florian.fainelli@broadcom.com>

Documentation/devicetree/bindings/soc/bcm/brcm,bcm23550-cdc.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/soc/bcm/brcm,bcm23550-cdc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Broadcom BCM23550 Cluster Dormant Control
+
+description:
+  The Cluster Dormant Control block keeps the CPU in idle state. A command
+  needs to be sent to this block to bring the CPU into running state.
+
+maintainers:
+  - Florian Fainelli <f.fainelli@gmail.com>
+
+properties:
+  compatible:
+    const: brcm,bcm23550-cdc
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    cdc@3fe0e000 {
+        compatible = "brcm,bcm23550-cdc";
+        reg = <0x3fe0e000 0x78>;
+    };
+...