dt-bindings: watchdog: sunxi: fix error in schema

"maxItems" is not needed with an "items" list

Fixes:
$ DT_SCHEMA_FILES=Documentation/devicetree/bindings/watchdog/allwinner,sun4i-a10-wdt.yaml make dtbs_check
Documentation/devicetree/bindings/watchdog/allwinner,sun4i-a10-wdt.yaml: properties:clocks: {'required': ['maxItems']} is not allowed for {'minItems': 1, 'maxItems': 2, 'items': [{'description': 'High-frequency oscillator input, divided internally'}, {'description': 'Low-frequency oscillator input, only found on some variants'}]}
	hint: "maxItems" is not needed with an "items" list
	from schema $id: http://devicetree.org/meta-schemas/items.yaml#


...

Signed-off-by: David Heidelberg <david@ixit.cz>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211029142443.68779-1-david@ixit.cz