drivers: nrf: Adjust clock_control and timer drivers for nRF9160

Minor adjustments are done to the nRF clock_control and rtc_timer
drivers to make them usable on nRF9160 as well.
The arm_irq_vector_table test code is modified only because it uses
the function that has been renamed in the nrf_rtc_timer driver.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>