gatt: Allow to set eatt support

With this patch it is possible to enable eatt_support as a GATT Client
or GATT Server.

Tag: #feature
Bug: 159786353
Test: manually verified against device supporting EATT
Sponsor: jpawlowski@
Change-Id: I6835a2bbd1b0ab9d6d64ee2bac5dfc96c0563afd