Bluetooth: controller: symbolic name for CC used in ISR profiling

This commit adds a symbolic name for the event timer
CC offset that is used to sample the event timer during
ISR profiling.

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