iio: st_sensors: move regulator retrieveal to core

Currently the pressure sensor has code to retrieve and enable two
regulators for Vdd and Vdd IO, but actually these voltage inputs
are found on all of these ST sensors, so move the regulator
handling to the core and make sure all the ST sensors call these
functions on probe() and remove() to enable/disable power.

Here also mover over to obtaining the regulator from the *parent*
device of the IIO device, as the IIO device is created on-the-fly
in this very subsystem it very unlikely evert have any regulators
attached to it whatsoever. It is much more likely that the parent
is a platform device, possibly instantiated from a device tree,
which in turn have Vdd and Vdd IO supplied assigned to it.

Cc: Lee Jones <lee.jones@linaro.org>
Cc: Denis CIOCCA <denis.ciocca@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>