Timer: Add TICKLESS_KERNEL support for nrf_rtc_timer.
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 <7f33386d52951c3e7a699b9509267436d9be79f8@intel.com>

finikorg/zephyr,rsalveti/zephyr,GiulianoFranchetto/zephyr,fbsder/zephyr,kraj/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,aceofall/zephyr-iotos,galak/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,aceofall/zephyr-iotos,ldts/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,galak/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,kraj/zephyr,Vudentz/zephyr,zephyriot/zephyr,kraj/zephyr,aceofall/zephyr-iotos,ldts/zephyr,zephyriot/zephyr,kraj/zephyr,zephyriot/zephyr,galak/zephyr,punitvara/zephyr,mbolivar/zephyr,nashif/zephyr,Vudentz/zephyr,aceofall/zephyr-iotos,kraj/zephyr,punitvara/zephyr,fbsder/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,explora26/zephyr,explora26/zephyr,nashif/zephyr,Vudentz/zephyr,fbsder/zephyr,runchip/zephyr-cc3220,runchip/zephyr-cc3220,zephyriot/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,fbsder/zephyr,mbolivar/zephyr,finikorg/zephyr,explora26/zephyr,runchip/zephyr-cc3220,GiulianoFranchetto/zephyr,Vudentz/zephyr,rsalveti/zephyr,galak/zephyr,runchip/zephyr-cc3220,galak/zephyr,nashif/zephyr,mbolivar/zephyr,explora26/zephyr,rsalveti/zephyr,mbolivar/zephyr,ldts/zephyr,ldts/zephyr,rsalveti/zephyr,nashif/zephyr,rsalveti/zephyr,fbsder/zephyr,runchip/zephyr-cc3220