dt-bindings: rtc: Add ST m41t93

Document DT bindings for m41t93 rtc which supports time, date,
alarm, watchdog, square wave clock output provider, user sram
and 8 bit timer.

Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/b1b1f7abde8a4ee8b625361bca32934679bf1e1a.1758379856.git.akhilesh@ee.iitb.ac.in
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/st,m41t93.yaml

@@ -0,0 +1,50 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/rtc/st,m41t93.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ST M41T93 RTC and compatible
+
+maintainers:
+  - Akhilesh Patil <akhilesh@ee.iitb.ac.in>
+
+description:
+  ST M41T93 is spi based Real Time Clock (RTC) with time, date,
+  alarm, watchdog, square wave clock output, 8 bit timer and
+  7 bytes of user SRAM.
+
+properties:
+  compatible:
+    enum:
+      - st,m41t93
+
+  reg:
+    maxItems: 1
+
+  "#clock-cells":
+    const: 0
+
+required:
+  - compatible
+  - reg
+
+allOf:
+  - $ref: rtc.yaml
+  - $ref: /schemas/spi/spi-peripheral-props.yaml#
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    spi {
+      #address-cells = <1>;
+      #size-cells = <0>;
+      rtc@0 {
+        compatible = "st,m41t93";
+        reg = <0>;
+        #clock-cells = <0>;
+        spi-max-frequency = <2000000>;
+      };
+    };
+