dt-bindings: hwlock: sun6i: Add #hwlock-cells to example

The dt-bindings tools will compile the yaml dt examples
and this prevents an error about this node not existing.

Signed-off-by: Bastian Germann <bage@debian.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230215203711.6293-3-bage@debian.org