drivers: stepper: Renamed Fault Event + Non Static Event Trigger

Renamed fault event to be more in line with other events and added it to
the stepper shell.
Makes the event callback trigger function of the step-dir implementation
non-static so that step-dir stepper drivers can use it to trigger events
themself.

Signed-off-by: Jan Behrens <jan.behrens@navimatix.de>