Have a look here: http://www.cisco.com/c/en/us/support/docs/wireless/aironet-3800-series-access-points/200898-Link-aggregation-on-1800-2800-3800-AP.html
I haven't read it, so I don't know if it's actually supported.
Just found this in the 8.2.151.0 release notes:
http://www.cisco.com/c/en/us/td/docs/wireless/controller/release/notes/crn82mr5.html#pgfId-1334529
Note Even if the Cisco AP is 802.3ad (LACP)-compliant, link aggregation groups (LAG) are not supported on the AP while it has a Cisco Mobility Express software image.