cpu/arm/k60: Protect _sbrk_r with a lock.

Added call to lock_acquire() in sbrk. sbrk is never safe to be called
from ISRs. If you call malloc or sbrk from an ISR there is a risk of
a deadlock if anyone else is already executing an sbrk.

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