cpu/stm32f0: optimized GPIO driver implementation

Tried a different concept for handling the GPIO-pin-and-port mapping (thx to @HexLuthor for the idea!).

The code is tested with `test/periph_gpio` and `tests/periph_gpio_int` and seems to work as before. The importent thing is the memory usage:

before applying this PR for `tests/periph_gpio`:
```
   text    data     bss     dec     hex 
  17440     112    4280   21832    5548
```

and after:
```
   text    data     bss     dec     hex 
  16428     112    4280   20820    5154
```

--> saves 1Kb of flash and makes the code a little less ugly...
`NVIC_SetPriority()` is not invoked anymore.