The port now uses the common mp_utime_ticks_{ms,us,cpu,add,diff} functions
from extmod/utime_mphal.c.

The mp_utime_sleep_XXX functions are still cc3200-specific because they
handle the GIL differently to the ones in extmod.

The files misc/mpsystick.[ch] have been removed because they contain 2
unused functions, and the other remaining function is renamed to
mp_hal_ticks_us and moved to hal/cc3200_hal.c.