meta-amd: 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-amd rev: 10e0f894f2bce5d131ecc4befce524bf432448c0)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ic852613d8ed9b8255228cc787b365dd66e30e3c4
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>