drivers: improved IF for low-level GPIO driver

Improved the GPIO interface and adjusted existing implementations to it.

Tested for
- stm32f0discovery
- stm32f4discovery

Not (much) tested for `arduino-due`, as proper implementation for it will follow later tonight in a separate PR.
I know, but it is specified in the vendor header files for this uC. And I didn't feel like touching some thousand lines of code just to make them comply with our Coding Conventions. 

I strongly say that using those vendor headers 'as-is' is a good idea!