dt-bindings: hwlock: sun6i: Add missing #hwlock-cells

The allwinner,sun6i-a31-hwspinlock.yaml binding needs #hwlock-cells
which is required by the hwlock type.

Fixes: f9e784dc ("dt-bindings: hwlock: add sun6i_hwspinlock")
Signed-off-by: Bastian Germann <bage@debian.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230213231931.6546-2-bage@debian.org