drivers: regulator: drop async enable

Drop the async enable function. This feature is rarely/never used,
complicates driver design, and doesn't really follow the sync/async API
design/naming used in other areas. In the future we can introduce
regulator_enable_async if needed, with support from the driver class (no
onoff). Note that drivers like PCA9420 did not implement any
asynchronous behavior. regulator-fixed implemented in the past
asynchronous behavior using work queues, an overkill for most GPIO
driven regulators. Let's keep things simple for now and extend the API
when needed, based on specific usecases.

In the current implementation, reference counting is managed by the
driver class. \isr-ok attribute is dropped, since calls are potentially
blocking. Note that drivers like PCA9420 already violated such rule.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>