drivers: timer: add RTC support as system clock for CC13X2/CC26X2

Add RTC timer driver for CC13X2/CC26X2, and use it instead of systick
as system clock. It is necessary to use this timer for power
management support, so that the system can exit from deep sleep upon
expiry of timeouts.

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>