meta-portwell: 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).

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I447787084f77abbac5c55896aaffcf648b100001
diff --git a/meta-neptune/conf/layer.conf b/meta-neptune/conf/layer.conf
index 5e44606..4addb78 100644
--- a/meta-neptune/conf/layer.conf
+++ b/meta-neptune/conf/layer.conf
@@ -7,3 +7,5 @@
 
 BBFILE_COLLECTIONS += "neptune"
 BBFILE_PATTERN_neptune = ""
+
+LAYERSERIES_COMPAT_neptune = "dunfell gatesgarth"