cfg80211: indicate support for external authentication

Define macro to indicate backport support for
external authentication where authentication can be
offloaded to userspace in specific cases such as SAE.

Change-Id: Ib253b303e82f583f61bc13d14c8d491d5ea2af15
CRs-Fixed: 2182553
Signed-off-by: Srinivas Dasari <dasaris@codeaurora.org>
Signed-off-by: Jiachao Wu <jiacwu@codeaurora.org>
Signed-off-by: Min Liu <minliu@codeaurora.org>