cpu/arm/k60: Add missing PIT_ticks_per_usec

Added missing declaration of PIT_ticks_per_usec to rev 1.x header files.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
Merge pull request #32 from gebart/for-mulle/rev1-add-missing-PIT-var

cpu/arm/k60: Add missing PIT_ticks_per_usec