dt-bindings: intel: document Agilex based board compatibles

Add new compatible for Agilex based boards.
Signed-off-by: NKrzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: NDinh Nguyen <dinguyen@kernel.org>

Documentation/devicetree/bindings/arm/intel,socfpga.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/intel,socfpga.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Intel SoCFPGA platform device tree bindings
+
+maintainers:
+  - Dinh Nguyen <dinguyen@kernel.org>
+
+properties:
+  $nodename:
+    const: "/"
+  compatible:
+    oneOf:
+      - description: AgileX boards
+        items:
+          - enum:
+              - intel,n5x-socdk
+              - intel,socfpga-agilex-socdk
+          - const: intel,socfpga-agilex
+
+additionalProperties: true
+
+...