iio:magnetometer:ak8975: power regulator support

Add support for an optional regulator which, if found into device-tree,
will power on device at probing time.
The regulator is declared into ak8975 DTS entry as a "vdd-supply" property.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>