dt-bindings: timer: add Ralink SoCs system tick counter

Add YAML doc for the system tick counter which is present on Ralink SoCs.

cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20231212093443.1898591-1-sergio.paracuellos@gmail.com

Documentation/devicetree/bindings/timer/ralink,cevt-systick.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/timer/ralink,cevt-systick.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: System tick counter present in Ralink family SoCs
+
+maintainers:
+  - Sergio Paracuellos <sergio.paracuellos@gmail.com>
+
+properties:
+  compatible:
+    const: ralink,cevt-systick
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - interrupts
+
+additionalProperties: false
+
+examples:
+  - |
+    systick@d00 {
+        compatible = "ralink,cevt-systick";
+        reg = <0xd00 0x10>;
+
+        interrupt-parent = <&cpuintc>;
+        interrupts = <7>;
+    };
+...