regulator: fixed: add support for under-voltage IRQ

Add interrupt support for under-voltage notification. This functionality
can be used on systems capable to detect under-voltage state and having
enough capacity to let the SoC do some emergency preparation.

This change enforce default policy to shutdown system as soon as
interrupt is triggered.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20231025084614.3092295-6-o.rempel@pengutronix.de


Signed-off-by: Mark Brown <broonie@kernel.org>