drivers: improved IF for low-level GPIO driver

Improved the GPIO interface and adjusted existing implementations to it.

Tested for
- stm32f0discovery
- stm32f3discovery
- stm32f4discovery
- pca1000[0|5]

Not (much) tested for `arduino-due`, as proper implementation for it will follow later tonight in a separate PR.
Here is what we do for now: i merge this PR (as others are waiting on it), but continue this discussion in #1581 and act on this if needed.