Bluetooth: controller: implement sw-switch with the event timer

This commit contributes the implementation of the sw-switch for
tIFS in nRF5 Radio using the EVENT_TIMER instead of a dedicated
TIMER instance. A Kconfig configuration is added so the user can
select whether to use the EVENT_TIMER for the tIFS switch or use
a dedicated TIMER instance.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>