spi: dt-bindings: Document the IBM FSI-attached SPI controller

IBM Power processors have a SPI controller that can be accessed
over FSI from a service processor. Document it.
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://msgid.link/r/20240514192630.152747-1-eajames@linux.ibm.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/ibm,spi-fsi.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/spi/ibm,spi-fsi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: IBM FSI-attached SPI Controller
+
+maintainers:
+  - Eddie James <eajames@linux.ibm.com>
+
+description:
+  A SPI controller found on IBM Power processors, accessed over FSI from a
+  service processor. This node will always be a child node of an ibm,fsi2spi
+  node.
+
+properties:
+  compatible:
+    enum:
+      - ibm,spi-fsi
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+allOf:
+  - $ref: spi-controller.yaml#
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    fsi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        spi@0 {
+            compatible = "ibm,spi-fsi";
+            reg = <0>;
+            #address-cells = <1>;
+            #size-cells = <0>;
+
+            eeprom@0 {
+                compatible = "atmel,at25";
+                reg = <0>;
+                size = <0x80000>;
+                address-width = <24>;
+                pagesize = <256>;
+                spi-max-frequency = <1000000>;
+            };
+        };
+    };