Fixed implicit function declaration warnings.
platform/mulle: Add missing include.

Added string.h for strlen().

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Fix compiler warnings in power-modes.c

False warnings about unused variables when doing dummy reads from
power control register:

warning: variable ‘dummy’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>