dt-bindings: i2c: google,cros-ec-i2c-tunnel: correct path to i2c-controller schema

The referenced i2c-controller.yaml schema is provided by dtschema
package (outside of Linux kernel), so use full path to reference it.

Cc: [e-mail]
Fixes: 1acd4577a66f ("dt-bindings: i2c: convert i2c-cros-ec-tunnel to json-schema")
Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Signed-off-by: Andi Shyti <[e-mail]>

Documentation/devicetree/bindings/i2c/google,cros-ec-i2c-tunnel.yaml

···
   google,cros-ec-spi or google,cros-ec-i2c.
 
 allOf:
-  - $ref: i2c-controller.yaml#
+  - $ref: /schemas/i2c/i2c-controller.yaml#
 
 properties:
   compatible: