dt-bindings: i2c: qcom,i2c-qup: Document power-domains

Similar to qcom,geni-i2c, for i2c-qup we need to vote for performance
states on the VDDCX power domain to ensure that required clock rates
can be generated correctly.

I2C is typically used with a fixed clock rate, so a single required-opp
is sufficient without a full OPP table (unlike spi-qup for example).

Signed-off-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20231128-i2c-qup-dvfs-v1-1-59a0e3039111@kernkonzept.com