This is a rewrite of the Kinetis GPIO driver which follows the
refactored API in [1]. Pins are specified using the GPIO_PIN(PORT_x, y)
macro, e.g. GPIO_PIN(PORT_E, 25) for the PTE25 pin.
The interrupt pin handling is now implemented as a linked list, this
is more memory efficient, but with a minor variation in interrupt
latency depending on in what order the pins were initialized at
runtime.
Because the linked list entries are taken from a shared pool, there is
also the possibility of running out of available configuration slots,
define the preprocessor macro GPIO_INT_POOL_SIZE in periph_conf.h if
you need more than 16 pins configured for interrupts in the same
application.
[1]: https://github.com/RIOT-OS/RIOT/pull/3095
