dt-bindings: rtc: Add Arm PL031 schema

The PL031 RTC binding has been in use for a long time, but never got
documented. Fortunately, it's simple and existing users are consistent.

Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: linux-rtc@vger.kernel.org
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210607193718.3090202-1-robh@kernel.org/

Documentation/devicetree/bindings/rtc/arm,pl031.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/rtc/arm,pl031.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Arm Primecell PL031 Real Time Clock
+
+select:
+  properties:
+    compatible:
+      contains:
+        const: arm,pl031
+  required:
+    - compatible
+
+allOf:
+  - $ref: rtc.yaml#
+
+maintainers:
+  - Rob Herring <robh@kernel.org>
+
+properties:
+  compatible:
+    items:
+      - const: arm,pl031
+      - const: arm,primecell
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    maxItems: 1
+
+  start-year: true
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - clock-names
+
+additionalProperties: false
+
+examples:
+  - |
+    rtc@10017000 {
+        compatible = "arm,pl031", "arm,primecell";
+        reg = <0x10017000 0x1000>;
+        clocks = <&pclk>;
+        clock-names = "apb_pclk";
+    };