meta-openpower: 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-openpower rev: d6809d5e21b57fca14572a4a1a945f3caa1ef91b)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I168a928a18d8b5c6146cc615f7070bb98a06bceb
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/meta-openpower/conf/layer.conf b/meta-openpower/conf/layer.conf
index 134e5ef..57e0925 100644
--- a/meta-openpower/conf/layer.conf
+++ b/meta-openpower/conf/layer.conf
@@ -9,4 +9,4 @@
 BBFILE_PATTERN_openpower-layer := "^${LAYERDIR}/"
 LAYERVERSION_openpower-layer = "1"
 LAYERDEPENDS_openpower-layer = "core phosphor-layer aspeed-layer"
-LAYERSERIES_COMPAT_openpower-layer = "warrior zeus dunfell"
+LAYERSERIES_COMPAT_openpower-layer = "dunfell gatesgarth"