sys: timeutil: Move and rename K_TICKS macros

These macros depend on time.h (which is not included in clock.h)
and are not really kernel related, but translation from the kernel
tick representation into time_spec values.
Let's place them in timeutils with their related macros/functions.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>