kinetis: Refactor PWM periph
Initially supports only Mulles with serial number > 220 (due to missing MK60DN256ZVLL10 support in k60). See also: https://github.com/RIOT-OS/RIOT/wiki/Board%3A-Mulle Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>