dt-bindings: fsi: Document the IBM SBEFIFO engine

The SBEFIFO engine provides an interface to the POWER processor
Self Boot Engine (SBE).

Signed-off-by: Eddie James <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Reviewed-by: Andrew Jeffery <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

Documentation/devicetree/bindings/fsi/ibm,p9-sbefifo.yaml

···
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/fsi/ibm,p9-sbefifo.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: IBM FSI-attached SBEFIFO engine
+
+maintainers:
+  - Eddie James <[e-mail]>
+
+description:
+  The SBEFIFO is an FSI CFAM engine that provides an interface to the
+  POWER processor Self Boot Engine (SBE). This node will always be a child
+  of an FSI CFAM node.
+
+properties:
+  compatible:
+    enum:
+      - ibm,p9-sbefifo
+      - ibm,odyssey-sbefifo
+
+  reg:
+    items:
+      - description: FSI slave address
+
+  occ:
+    type: object
+    $ref: ibm,p9-occ.yaml#
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    fsi-slave-engine@2400 {
+        compatible = "ibm,p9-sbefifo";
+        reg = <0x2400 0x400>;
+
+        occ {
+            compatible = "ibm,p9-occ";
+        };
+    };