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 <[e-mail]>
Signed-off-by: Kunihiko Hayashi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/socionext,uniphier-uart.yaml

···
   clocks:
     maxItems: 1
 
+  resets:
+    maxItems: 1
+
   auto-flow-control:
     description: enable automatic flow control support.
     $ref: /schemas/types.yaml#/definitions/flag