The device lm90 can be controlled by the vcc rail.
Adding the regulator support to power on/off the vcc rail.
Enable the "vcc" regulator before accessing the device.

[JD: Rename variables to avoid confusion with registers.]
Signed-off-by: NWei Ni <wni@nvidia.com>
Signed-off-by: NJean Delvare <khali@linux-fr.org>