drivers - gpio: expanded gpio driver interface

- added functions for en|disabling a pins interrupt
driver - gpio: small adjustments
Merge pull request #1368 from haukepetersen/fix_gpio_disableinterrupt

drivers - gpio: expanded gpio driver interface
sys/ps: s/float/int for runtime

The high precision isn't needed, use per mille instead.
Circumvents printf problems on some platforms.

Also: prevent division by zero.
sys/ps: fix comment style, superfluous declaration