regulator: irq_helper: Provide helper for trivial IRQ notifications

Provide a generic map_event helper for regulators which have a notification
IRQ with single, well defined purpose. Eg, IRQ always indicates exactly one
event for exactly one regulator device. For such IRQs the mapping is
trivial.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/603b7ed1938013a00371c1e7ccc63dfb16982b87.1637736436.git.matti.vaittinen@fi.rohmeurope.com


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