dt-bindings: i2c: uniphier: Add resets property

UniPhier I2C controller allows reset control support.
Add resets property to the controller as optional.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>