dt-bindings: mtd: st,spear600-smi: Fix description

The description mixes two nodes. There is the controller, and there is
the flash. Describe the flash (which itself can be considered an mtd
device, unlike the top level controller), and move the st,smi-fast-mode
property inside, as this property is flash specific and should not live
in the parent controller node.

Fixes: 68cd8ef484 ("dt-bindings: mtd: st,spear600-smi: convert to DT schema")
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

Documentation/devicetree/bindings/mtd/st,spear600-smi.yaml

@@ -19,9 +19,6 @@ description:
   Flash sub nodes describe the memory range and optional per-flash
   properties.
 
-allOf:
-  - $ref: mtd.yaml#
-
 properties:
   compatible:
     const: st,spear600-smi
@@ -42,9 +39,22 @@ properties:
     $ref: /schemas/types.yaml#/definitions/uint32
     description: Functional clock rate of the SMI controller in Hz.
 
-  st,smi-fast-mode:
-    type: boolean
-    description: Indicates that the attached flash supports fast read mode.
+patternProperties:
+  "^flash@.*$":
+    $ref: /schemas/mtd/mtd.yaml#
+
+    properties:
+      reg:
+        maxItems: 1
+
+      st,smi-fast-mode:
+        type: boolean
+        description: Indicates that the attached flash supports fast read mode.
+
+    unevaluatedProperties: false
+
+    required:
+      - reg
 
 required:
   - compatible