regulator: userspace-consumer: Add regulator event support

Add sysfs attribute to track regulator events received from regulator
notifier block handler.

Signed-off-by: Naresh Solanki <Naresh.Solanki@9elements.com>
Link: https://lore.kernel.org/r/20230803111225.107572-1-Naresh.Solanki@9elements.com


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