dt-bindings: reset: Add parent "resets" property as optional

LD11 mio reset controller has a reset lines from system controller.
Add parent "resets" property to fix the following warning.

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

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://lore.kernel.org/r/1648617078-8312-1-git-send-email-hayashi.kunihiko@socionext.com