cpu/arm/k60: Implement clock via rtimer API.

 - Change rtimer to use LPTMR (support STOP modes while rtimer is active)
 - Change clock module to use rtimer (instead of LPTMR hardware)
 - Reduce rtimer_clock_t to 32 bits (was a hassle to handle 64 bit)
 - Reduce RTIMER_SECOND to 32768 (was F_BUS == 48000000)
 - Handle the RTIMER_PERIOD overflow correctly.
 - Handle timers set beyond the reach of the LPTMR.

LPTMR is only 16 bits wide and we handle rtimers beyond the 2 second limit
of the hardware timer by keeping track of a 32 bit offset.

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