media: dt-bindings: qcom,sm8250-venus: document OPP table

Venus on Qualcomm SM8250 uses Operating Performance Points (both in DTS
and driver):

  sm8250-hdk.dtb: video-codec@aa00000: 'operating-points-v2', 'opp-table' do not match any of the regexes: 'pinctrl-[0-9]+'

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>