Timer: Add TICKLESS_KERNEL support for nrf_rtc_timer.

Add support for TICKLESS KERNEL in nrf_rtc_timer for Nordic family of
processors. This patch includes :-
 1. Programming RTC based on "next timer" value from timeout queue while
     A. exiting idle_exit.
     B. on RTC interrupt.
 2. Impliments some of functions which will be required by sys_clock and
    scheduler.

ZEP-1819

Signed-off-by: Youvedeep Singh <youvedeep.singh@intel.com>