dt-bindings: display: msm/dp: 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>
Link: https://lore.kernel.org/r/20230820080543.25204-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>