meta-arm: 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-arm rev: e412371fb34b85c28eddc829359da422eae3489f)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ieb12f2b8b48c047a6409b317a876e0b326c14f2e
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>