regulator: Add device tree support to AD5398

regulator: Add device tree support to AD5398

Merge series from Isaac Scott <isaac.scott@ideasonboard.com>:

The AD5398 is a DAC that can be used to control current flow in circuits
in a wide variety of applications such as motor control, or in my case,
LED control. I found when working with the current driver that it did
not work for my use case. It transpired that it only had support for
platform_data, and didn't appear to be correctly implemented according
to the datasheet, which can be found here:

https://www.analog.com/media/en/technical-documentation/data-sheets/ad5398.pdf

One example of this is the "soft power-down" bit being referred to in
the driver as simply "enable", which gives the impression that the
setting that bit will allow current through the regulator, which it does
not.

This series allows the regulator to be given its constraints via the
device tree, and makes the function of the enable register much more
obvious.