dt-bindings: fsi: Add FSI2SPI bindings

Add documentation for the FSI2SPI CFAM engine, which provides access to
a number of SPI controllers.
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20200306194118.18581-2-eajames@linux.ibm.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/fsi/ibm,fsi2spi.yaml

+# SPDX-License-Identifier: (GPL-2.0-or-later)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/fsi/ibm,fsi2spi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: IBM FSI-attached SPI controllers
+
+maintainers:
+ - Eddie James <eajames@linux.ibm.com>
+
+description: |
+  This binding describes an FSI CFAM engine called the FSI2SPI. Therefore this
+  node will always be a child of an FSI CFAM node; see fsi.txt for details on
+  FSI slave and CFAM nodes. This FSI2SPI engine provides access to a number of
+  SPI controllers.
+
+properties:
+  compatible:
+    enum:
+      - ibm,fsi2spi
+
+  reg:
+    items:
+      - description: FSI slave address
+
+required:
+  - compatible
+  - reg
+
+examples:
+  - |
+    fsi2spi@1c00 {
+        compatible = "ibm,fsi2spi";
+        reg = <0x1c00 0x400>;
+    };