drivers: timer: nRF SoC Series RTC system clock implementation

The nRF5x series SoCs do not implement systick, hence we disable
CORTEX_M_SYSTICK.
Instead, use nRF SoC Series NRF_RTC1 for system clock interfaces.
The kernel system clock interface is implemented using the low
power real time counter NRF_RTC1. NRF_RTC0 is used by the BLE
controller.

In addition, cleanup nRF5x series defconfig to be consistent.

Jira: ZEP-742
Jira: ZEP-1308
Jira: ZEP-1315

Change-id: I0f6cc1836fe0820a65f2cbb02cf5ae7e9eb92e1d
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>