Bluetooth: controller: Kconfig for advanced event preparation

Added Kconfig option to enable advanced event preparation
feature.

Enables advanced event preparation offset ahead of radio
tx/rx, taking into account predictive processing time
requirements in preparation to the event, like control
procedure handling and CPU execution speeds. Crystal
oscillator is retained between closely spaced consecutive
radio events to reduce the overall number of crystal
settling current consumptions.

This feature maximizes radio utilization in an average role
event timeslice when they are closely spaced by using a
reduced offset between preparation and radio event.

By disabling this feature, the controller will use a
constant offset between the preparation and radio event. The
controller will toggle crystal oscillator between two
closely spaced radio events leading to higher average
current due to increased number of crystal settling current
consumptions.

Change-id: I19e640f7395ac7938873ef4bfac38acf8d6f7e0e
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>