dt-bindings: serial: uniphier: Use maxItems for a single clock

'maxItems' is preferred for a single entry, not 'minItems'.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/1648617338-8600-2-git-send-email-hayashi.kunihiko@socionext.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>