Bluetooth: Controller: Introduce EVENT_MAFS_MIN_US value

Introduce Controller dependent EVENT_MAFS_MIN_US value to
use as the value when populating the aux offset between
Extended Advertising primary and auxiliary channel PDUs.

This can be used as workaround for peers having difficulty
receiving Extended Advertising PDUs with near 300 us MAFS
values used in aux offset calculations.

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