drivers: stepper: step_dir: Add fallback for dual-edge-step

Adds a fallback for the dual-edge-step property in the step dir common
code. Without this drivers using the common code would have to declare the
dual-edge-step property so it can default to false when not set, even if
the IC does not support adjusting it.

Signed-off-by: Fabian Blatz <fabianblatz@gmail.com>