dt-bindings: arm: intel,keembay: limit the dtschema to root node

The check for the board compatible should be limited only to the root
node.  Any other nodes with such compatible are not part of this schema
and should not match.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Daniele Alessandrelli <daniele.alessandrelli@intel.com>
Link: https://lore.kernel.org/r/20210526162750.135139-1-krzysztof.kozlowski@canonical.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/intel,keembay.yaml

@@ -11,6 +11,8 @@ maintainers:
   - Daniele Alessandrelli <daniele.alessandrelli@intel.com>
 
 properties:
+  $nodename:
+    const: '/'
   compatible:
     items:
       - enum: