Bluetooth: Controller: Kconfig fast encryption setup feature

Enable connection encryption setup in 3 connection
intervals. Peripheral will respond to Encryption Request
with Encryption Response in the same connection interval,
and also, will respond with Start Encryption Response PDU in
the 3rd connection interval, hence completing encryption
setup in 3 connection intervals. Encrypted data would be
transmitted as fast as in 3rd connection interval from the
connection establishment. Maximum CPU time in Radio ISR will
increase if this feature is selected.

Change-id: I16f2863fc2aaed624826505739519d4de1ac44c5
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>