iio: tsl2772: add support for regulator framework

This patch adds support for the regulator framework to the tsl2772
driver. Driver was tested using a LG Nexus 5 (hammerhead) phone with
the two regulators and on a Raspberry Pi 2 without any regulators
controlling the power to the sensor.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>