drivers: regulator: common: allow to specify on by default

Some regulators are enabled by default, however, such condition cannot
be captured now by the regulator driver API. Refactor
regulator_common_init_enable to regulator_common_init (enable removed,
as it also sets mode) and add a new argument to specify such condition.
With this change, regulator_disable() and regulator_is_enabled() work as
expected without a first call to regulator_enable().

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