iio: gyro: bmg160: Add rudimentary regulator support

BMG160 needs VDD and VDDIO regulators that might need to be explicitly
enabled. Add some rudimentary support to obtain and enable these
regulators during probe() and disable them using a devm action.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20201211183815.51269-2-stephan@gerhold.net


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>