dt-bindings: rtc: Add Mstar MSC313e RTC devicetree bindings documentation

This adds the documentation for the devicetree bindings of the Mstar
MSC313e RTC driver, found from MSC313e SoCs and newer.
Signed-off-by: Romain Perier <romain.perier@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210823171613.18941-2-romain.perier@gmail.com

Documentation/devicetree/bindings/rtc/mstar,msc313-rtc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/rtc/mstar,msc313-rtc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Mstar MSC313e RTC Device Tree Bindings
+
+allOf:
+  - $ref: "rtc.yaml#"
+
+maintainers:
+  - Daniel Palmer <daniel@0x0f.com>
+  - Romain Perier <romain.perier@gmail.com>
+
+properties:
+  compatible:
+    enum:
+      - mstar,msc313-rtc
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  start-year: true
+
+  clocks:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - clocks
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    rtc@2400 {
+        compatible = "mstar,msc313-rtc";
+        reg = <0x2400 0x40>;
+        clocks = <&xtal_div2>;
+        interrupts-extended = <&intc_irq GIC_SPI 44 IRQ_TYPE_LEVEL_HIGH>;
+    };
+...