iio: pressure-core: st: Provide support for the Vdd power supply

The power to some of the sensors are controlled by regulators. In most
cases these are 'always on', but if not they will fail to work until
the regulator is enabled using the relevant APIs. This patch allows for
the Vdd power supply to be specified by either platform data or Device
Tree.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>