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: 053aaa505b711929800bffdcc1f748ddb89e50e7)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I98a0b761a51db8813ac50fb89ba3923b89a39259
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>