meta-qualcomm: 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-qualcomm rev: 975a7515eb7898718f9b883b4fc6e6a632640ab2)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ib572286f5ec36da446deea21bb70c21298d71137
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>