dt-bindings: gpu: arm,mali: restrict opp-table to objects

Simple 'opp-table:true' accepts a boolean property as opp-table, so
restrict it to object to properly enforce real OPP table nodes.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220818061713.9611-1-krzysztof.kozlowski@linaro.org