kinetis: Refactor GPIO to use a look-up-table

Saves a bunch of ROM space

mulle, gcc-4.9.2:
   text    data     bss     dec     hex filename
  20800     216   65320   86336   15140 /data/riotbuild/riotproject/tests/periph_gpio/bin/mulle/periph_gpio.elf
  18992     216   65320   84528   14a30 /data/riotbuild/riotproject/tests/periph_gpio/bin/mulle/periph_gpio.elf
  diff = 1808
  36080     272   65264  101616   18cf0 /data/riotbuild/riotproject/examples/default/bin/mulle/default.elf
  34296     272   65264   99832   185f8 /data/riotbuild/riotproject/examples/default/bin/mulle/default.elf
  diff = 1784