Change the stepper API to instead of changing the stepper speed based on
the velocity in microsteps per second to use the delay in usec between
successive steps. Also remove the velocity from the `stepper_run` function
as typical API usage is enable -> set step interval -> run.
Signed-off-by: Fabian Blatz <fabianblatz@gmail.com>
Adds a timing source api which is used by the step-dir stepper common code.
This allows the reusable common code to configure different timing sources,
since the initial delayable work implementation was inacurate for higher
maximum velocities.
Signed-off-by: Fabian Blatz <fabianblatz@gmail.com>