cpu/arm/k60: Implement rtimer_arch API.

Implemented by using a single PIT channel and a 64 bit timer offset for
full RTIMER_NOW functionality. There may be some slight drift due to
a few lost ticks during the rtimer ISR.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: BITBAND_REG in clock.c

Added more bitbanding.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Remove redundant RTC initialization.

clock_init should not be called without first initializing the RTC
module. This is done by core_clocks_init_early during boot up.

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