dt-bindings: media: venus: Add an optional power domain for perf voting

Venus needs to vote for the performance state of a power domain (cx)
to be able to support DVFS. This 'cx' power domain is controlled by
rpmh and is a common power domain (scalable) not specific to
venus alone. This is optional in the sense that, leaving this power
domain out does not really impact the functionality but just makes
the platform a little less power efficient.

Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>