media: dt-bindings: qcom,sc7180-venus: document OPP table

Venus on Qualcomm SC7180 uses Operating Performance Points (both in DTS
and driver):

  sc7180-idp.dtb: video-codec@aa00000: Unevaluated properties are not allowed ('operating-points-v2', 'opp-table' were unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>