dt-bindings: i2c: atmel,at91sam: 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: stable@vger.kernel.org
Fixes: 7ea75dd3

 ("dt-bindings: i2c: convert i2c-at91 to json-schema")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>