mac80211: sync method beacon alignment

This synchronization method aligns TBTTs in a narrow time
frame to reduce wakeup overheads in light sleep mode.
Nodes using this sync method will determine the TBTT offsets
between each other. Depending on the own and peers' offsets
the "steady node" is determined. The steady node is the node
with the highest TBTT offset towards the next later TBTT.
Subsequently, all nodes except the steady node will gradually
delay their TBTT, thus shifting towards the next later TBTT.
Once a predefined offset towards this TBTT is reached, it
stops shifting any further. Finally, one or multiple aligned
wakeup groups are formed. Light sleep nodes benefit from
waking up less times to catch all peer beacons. Since the
wakeup overhead of powering up receivers and waiting for the
beacon is avoided, the power consumption is reduced.

Signed-off-by: Marco Porsch <marco@cozybit.com>