regulator: new API for voltage reference supplies

regulator: new API for voltage reference supplies

Merge series from David Lechner <dlechner@baylibre.com>:

In the IIO subsystem, we noticed a pattern in many drivers where we need
to get, enable and get the voltage of a supply that provides a reference
voltage. In these cases, we only need the voltage and not a handle to
the regulator. Another common pattern is for chips to have an internal
reference voltage that is used when an external reference is not
available. There are also a few drivers outside of IIO that do the same.

So we would like to propose a new regulator consumer API to handle these
specific cases to avoid repeating the same boilerplate code in multiple
drivers.

As an example of how these functions are used, I have included a few
patches to consumer drivers. But to avoid a giant patch bomb, I have
omitted the iio/adc and iio/dac patches I have prepared from this
series. I will send those separately but these will add 36 more users
of devm_regulator_get_enable_read_voltage() in addition to the 6 here.
In total, this will eliminate nearly 1000 lines of similar code and will
simplify writing and reviewing new drivers in the future.