drivers/timers/arch_timer.c: implement timer arch API on top of timer driver interface
drivers/timers/arch_alarm.c: Implement alarm arch API on top of oneshot driver interface
drivers/timers/arch_rtc.c: Implement RTC arch API on top of RTC driver interface
include/nuttx/timers/rtc.h: Extend struct rtc_time by adding tm_nsec if RTC supporst hiresolution time.