Bluetooth: Controller: add radio ISR profiling event

Added code to profile radio ISR execution time and generate
an event, which can be used to populate a vendor HCI event.

This event and associated code is disabled, subsequent
patch will add kconfig to enable it.

Change-id: Ic3fa3e0f4e36829a22a25ffee039949eaae561a7
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>