dt-bindings: rtc: Add ST M48T86

Add YAML bindings for ST M48T86 / Dallas DS12887 RTC.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/lkml/61b9e036-7864-65c6-d43b-463fff896ddc@linaro.org/
Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Link: https://lore.kernel.org/r/20230823-m48t86_device_tree-v2-1-21ff275f949d@maquefel.me
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/st,m48t86.yaml

@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/rtc/st,m48t86.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ST M48T86 / Dallas DS12887 RTC with SRAM
+
+maintainers:
+  - Alexandre Belloni <alexandre.belloni@bootlin.com>
+
+allOf:
+  - $ref: rtc.yaml
+
+properties:
+  compatible:
+    enum:
+      - st,m48t86
+
+  reg:
+    items:
+      - description: index register
+      - description: data register
+
+required:
+  - compatible
+  - reg
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    rtc@10800000 {
+      compatible = "st,m48t86";
+      reg = <0x10800000 0x1>, <0x11700000 0x1>;
+    };
+
+...