dt-bindings: rtc: stmp3xxx-rtc: convert to dtschema

Convert existing binding to dtschema to support validation and
add the undocumented compatible 'fsl,imx23-rtc'.
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240413-rtc_dtschema-v3-4-eff368bcc471@gmail.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/fsl,stmp3xxx-rtc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/rtc/fsl,stmp3xxx-rtc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: STMP3xxx/i.MX28 Time Clock Controller
+
+maintainers:
+  - Javier Carrasco <javier.carrasco.cruz@gmail.com>
+
+allOf:
+  - $ref: rtc.yaml#
+
+properties:
+  compatible:
+    oneOf:
+      - items:
+          - enum:
+              - fsl,imx28-rtc
+              - fsl,imx23-rtc
+          - const: fsl,stmp3xxx-rtc
+      - const: fsl,stmp3xxx-rtc
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  stmp,crystal-freq:
+    description:
+      Override crystal frequency as determined from fuse bits.
+      Use <0> for "no crystal".
+    $ref: /schemas/types.yaml#/definitions/uint32
+    enum: [0, 32000, 32768]
+
+required:
+  - compatible
+  - reg
+  - interrupts
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    rtc@80056000 {
+        compatible = "fsl,imx28-rtc", "fsl,stmp3xxx-rtc";
+        reg = <0x80056000 2000>;
+        interrupts = <29>;
+    };

Documentation/devicetree/bindings/rtc/stmp3xxx-rtc.txt

-* STMP3xxx/i.MX28 Time Clock controller
-
-Required properties:
-- compatible: should be one of the following.
-    * "fsl,stmp3xxx-rtc"
-- reg: physical base address of the controller and length of memory mapped
-  region.
-- interrupts: rtc alarm interrupt
-
-Optional properties:
-- stmp,crystal-freq: override crystal frequency as determined from fuse bits.
-  Only <32000> and <32768> are possible for the hardware.  Use <0> for
-  "no crystal".
-
-Example:
-
-rtc@80056000 {
-	compatible = "fsl,imx28-rtc", "fsl,stmp3xxx-rtc";
-	reg = <0x80056000 2000>;
-	interrupts = <29>;
-};