dt-bindings: Add Broadcom AVS RO thermal

Since the BCM2711 doesn't have a AVS TMON block, the thermal information
must be retrieved from the AVS ring oscillator block. This block is part
of the AVS monitor which contains a bunch of raw sensors.
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1578941778-23321-2-git-send-email-stefan.wahren@i2se.com

Documentation/devicetree/bindings/thermal/brcm,avs-ro-thermal.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/thermal/brcm,avs-ro-thermal.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Broadcom AVS ring oscillator thermal
+
+maintainers:
+  - Stefan Wahren <wahrenst@gmx.net>
+
+description: |+
+  The thermal node should be the child of a syscon node with the
+  required property:
+
+  - compatible: Should be one of the following:
+                "brcm,bcm2711-avs-monitor", "syscon", "simple-mfd"
+
+  Refer to the the bindings described in
+  Documentation/devicetree/bindings/mfd/syscon.txt
+
+properties:
+  compatible:
+    const: brcm,bcm2711-thermal
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+examples:
+  - |
+        avs-monitor@7d5d2000 {
+                compatible = "brcm,bcm2711-avs-monitor",
+                             "syscon", "simple-mfd";
+                reg = <0x7d5d2000 0xf00>;
+
+                thermal: thermal {
+                        compatible = "brcm,bcm2711-thermal";
+                        #thermal-sensor-cells = <0>;
+                };
+        };
+...