nrf51822/timer: Add code to check for too long intervals
with the current setup, max time is ~16s
timer: Add get_remaining_us() function to API
nrf51822/timer: Implement get_remaining_us()
nrf51822/timer: fix bug concerning the number of active timers
nrf51822/timer: fix bug that caused invalid durations
when a timer is manually stopped, then fires (causes no IRQ), then is started
again, the compare event is still high for this channel and the timer will
fire on the next IRQ (caused by another channel).