Adding driver for AVSBus

[snematbakhsh]

AVSBus is a standardized way of controlling PMICs (intro: https://www.scanti.ru/images/products/renesas/news/20201119/2016-08AVSBusIntro.pdf). It is implemented on top of SPI. It is commonly used for:

• Setting output voltage / slew rate of a power rail.
• Reading status such as temperature and current draw.
• Reading fault events.

I have written some software routines and console commands for sending AVSBus commands and I would like to add this support to upstream Zephyr.

My initial question is related to where an AVSBus driver should live in the driver hierarchy. /drivers/regulator/ seems like a candidate, but the existing regulator driver methods aren't a good fit. Maybe it's a better idea to add a completely new driver interface (maybe in drivers/spi/spi_avsbus.{c,h})? Or maybe there another appropriate existing interface that I missed?

Thanks for the guidance.