iio: imu: bmi160: added regulator support

Add vdd-supply and vddio-supply support.

While working on an msm8916 device and having explicit declarations for
regulators, without setting these regulators to regulators-always-on it
happened those lines weren't ready because they could have been controlled
by other components, causing failure in module's probe.

This patch aim is to solve this situation by adding regulators control
during bmi160_chip_init() and bmi160_chip_uninit(), assuring power to
this component.

Signed-off-by: Jonathan Albrieux <jonathan.albrieux@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>