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.
Not really part of this PR, but this type name is not according to the [Coding Conventions](https://github.com/RIOT-OS/RIOT/wiki/Coding-conventions#types) (and https://www.kernel.org/doc/Documentation/CodingStyle, Chapter 4)