meta-phosphor: 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-phosphor rev: 49b2a2c56bb774224b15f80deeffff096f59a2db)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ie37ab65b72c2e10e0324774c56b7968db091ea5b
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/meta-phosphor/conf/layer.conf b/meta-phosphor/conf/layer.conf
index 028c6e5..2292acc 100644
--- a/meta-phosphor/conf/layer.conf
+++ b/meta-phosphor/conf/layer.conf
@@ -8,7 +8,7 @@
 BBFILE_COLLECTIONS += "phosphor-layer"
 BBFILE_PATTERN_phosphor-layer := "^${LAYERDIR}/"
 LAYERVERSION_phosphor-layer = "1"
-LAYERSERIES_COMPAT_phosphor-layer = "warrior zeus dunfell"
+LAYERSERIES_COMPAT_phosphor-layer = "dunfell gatesgarth"
 
 IMAGE_FEATURES[validitems] += "tools-profile"