Most of the functions for this was declared in a top level lk/ include
space, so go ahead and move it there.
A few exceptions:
- Moved spin() over to platform/time.h and platform/time.c since the
function more logically belongs to platform/time.h. Any users of
spin() will need to update their headers to include platform/time.h
instead.
- Renamed spin_cycles() to arm_cm_spin_cycles() and moved over into
arm/cm.h since it is currently defined in arch/arm-m and only used for
targets that implicitly are for arm-m.
