spi: Add Socionext F_OSPI controller bindings

Add devicetree binding documentation for Socionext F_OSPI SPI flash
controller.
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221124003351.7792-2-hayashi.kunihiko@socionext.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/socionext,f-ospi.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/spi/socionext,f-ospi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Socionext F_OSPI controller
+
+description: |
+  The Socionext F_OSPI is a controller used to interface with flash
+  memories using the SPI communication interface.
+
+maintainers:
+  - Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
+
+allOf:
+  - $ref: spi-controller.yaml#
+
+properties:
+  compatible:
+    const: socionext,f-ospi
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  num-cs:
+    minimum: 1
+    maximum: 4
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - "#address-cells"
+  - "#size-cells"
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    ospi0: spi@80000000 {
+        compatible = "socionext,f-ospi";
+        reg = <0x80000000 0x1000>;
+        clocks = <&clks 0>;
+        num-cs = <1>;
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        flash@0 {
+            compatible = "spansion,s25fl128s", "jedec,spi-nor";
+            reg = <0>;
+            spi-max-frequency = <50000000>;
+        };
+    };