Bluetooth: Controller: Add Kconfig for minimal time reservation

Add Kconfig to use minimal time reservation for auxiliary
and sync PDU reception. A peer device could be scheduling
multiple advertising sets in a short duration with small
PDUs hence using the minimal time reservation would avoid
skipping closely scheduled reception of multiple auxiliary
PDUs.

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