cpu/arm/k60: Add semaphores for inhibiting lower power modes.

Split allow_deep_sleep into three separate counters acting as
semaphores for signalling when to avoid lower power modes.

Drivers can use LLWU_INHIBIT_STOP() and LLWU_UNINHIBIT_STOP() to
increase/decrease the counters. Only if equal numbers of
INHIBIT/UNINHIBIT calls have been performed will the chip enter the
low power modes.

There are similar macros defined for inhibiting LLS and VLPS.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Use llwu semaphores to inhibit stop while still writing bytes

This fixes the problem with dropping the last byte when printing stuff
right before going to low power mode.

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