dt-bindings: i2c: qcom-cci: add description of a vbus-supply property

Quite regularly I2C bus lines on QCOM CCI controller require an external
pull-up to a regulator powered line, to be able to define all such
cases an additional vbus-supply property of a bus subnode is wanted.

Signed-off-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>