drivers/timer: Reworked NRF driver with tickless support

Reworked using the older hardware interface code, but with an
implementation of the new API only.  Much smaller & simpler.

As yet, tested (manually) only on a nrf52_pca10056 board.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

Author: Andy Ross

drivers/timer/Kconfig

@@ -152,6 +152,7 @@ config NRF_RTC_TIMER
 	bool "nRF Real Time Counter (NRF_RTC1) Timer"
 	default y
 	depends on CLOCK_CONTROL_NRF5
+	select TICKLESS_CAPABLE
 	help
 	  This module implements a kernel device driver for the nRF Real Time
 	  Counter NRF_RTC1 and provides the standard "system clock driver"