regulator: da9121: add interrupt support

Adds interrupt handler for variants, and notifications for events; over
temperature/voltage/current. Because the IRQs are triggered by persisting
status, they must be masked and the status polled until clear, before the
IRQ can be enabled again.
Signed-off-by: Adam Ward <Adam.Ward.opensource@diasemi.com>
Link: https://lore.kernel.org/r/fe21796bbcbadff84a472a4cc581ae8fafc7f8f5.1606755367.git.Adam.Ward.opensource@diasemi.comSigned-off-by: Mark Brown <broonie@kernel.org>