regulator: dt-bindings: fixed-regulator: Add under-voltage interrupt support

Add under-voltage interrupt support. This can be used with simple
regulators having no other way to communicate an under-voltage event
except as by toggling some GPIO line.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20231025084614.3092295-4-o.rempel@pengutronix.de


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