regulator: core: forward undervoltage events downstream by default

regulator: core: forward undervoltage events downstream by default

Forward critical supply events downstream so consumers can react in
time.  An under-voltage event on an upstream rail may otherwise never
reach end devices (e.g. eMMC).

Register a notifier on a regulator's supply when the supply is resolved,
and forward only REGULATOR_EVENT_UNDER_VOLTAGE to the consumer's notifier
chain. Event handling is deferred to process context via a workqueue; the
consumer rdev is lifetime-pinned and the rdev lock is held while calling
the notifier chain. The notifier is unregistered on regulator teardown.

No DT/UAPI changes. Behavior applies to all regulators with a supply.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20251001105650.2391477-1-o.rempel@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>