dt-bindings: serial: uniphier: Add "resets" property as optional

UniPhier UART controller has a reset lines from system controller.
Add "resets" property to fix the following warning.

  uniphier-ld11-global.dtb: serial@54006800: 'resets' does not match any of the regexes: 'pinctrl-[0-9]+'
      From schema: Documentation/devicetree/bindings/serial/socionext,uniphier-uart.yaml

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/1648617338-8600-3-git-send-email-hayashi.kunihiko@socionext.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>