meta-intel: update Yocto compat for gatesgarth

The next release of Yocto is soon and most of the upstream
layers have switched support strings for it.  Support layer
compat for gatesgarth (current) and dunsfell (previous).

(From meta-intel rev: 43deddce9ce3761a856deee4ebefa3ac922535d8)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I91cf833d32f64e2332a63e31971f824e66b8d92d
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>