dt-bindings: fsi: fsi2spi: Document SPI controller child nodes

The FSI2SPI bridge has several SPI controllers behind it, which
should be documented. Also, therefore the node needs to specify
address and size cells.

Signed-off-by: Eddie James <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

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

···
 maintainers:
   - Eddie James <[e-mail]>
 
-description: |
+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.
+  node will always be a child of an FSI CFAM node. This FSI2SPI engine provides
+  access to a number of SPI controllers.
 
 properties:
   compatible:
···
   reg:
     items:
       - description: FSI slave address
+
+  "#address-cells":
+    const: 1
+
+  "#size-cells":
+    const: 0
+
+patternProperties:
+  "^spi@[0-9a-f]+$":
+    type: object
+    $ref: /schemas/spi/ibm,spi-fsi.yaml
 
 required:
   - compatible
···
     fsi2spi@1c00 {
         compatible = "ibm,fsi2spi";
         reg = <0x1c00 0x400>;
+        #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>;
+                address-width = <24>;
+                pagesize = <256>;
+                size = <0x80000>;
+                spi-max-frequency = <1000000>;
+            };
+        };
     };