ARM: tegra: colibri_t30: annotate power I2C being on-module

The power I2C bus aka PWR_I2C which connects to the audio codec, PMIC,
temperature sensor and touch screen controller is really on-module only.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>