spi: dt-bindings: add documentation for hpe,gxp-spifi

Create documentation for the hpe,gxp-spifi binding to support access to
the SPI parts
Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220728161459.7738-3-nick.hawkins@hpe.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/hpe,gxp-spifi.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/spi/hpe,gxp-spifi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: HPE GXP spi controller flash interface
+
+maintainers:
+  - Nick Hawkins <nick.hawkins@hpe.com>
+  - Jean-Marie Verdun <verdun@hpe.com>
+
+allOf:
+  - $ref: spi-controller.yaml#
+
+properties:
+  compatible:
+    const: hpe,gxp-spifi
+
+  reg:
+    items:
+      - description: cfg registers
+      - description: data registers
+      - description: mapped memory
+
+  interrupts:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - interrupts
+
+unevaluatedProperties: false
+
+examples:
+  - |
+
+    spi@200 {
+      compatible = "hpe,gxp-spifi";
+      reg = <0x200 0x80>, <0xc000 0x100>, <0x38000000 0x800000>;
+      interrupts = <20>;
+      interrupt-parent = <&vic0>;
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      flash@0 {
+        reg = <0>;
+        compatible = "jedec,spi-nor";
+      };
+
+      flash@1 {
+        reg = <1>;
+        compatible = "jedec,spi-nor";
+      };
+    };