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).

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iba4f7ae3de5823d6798c2cdf452e485feecf38c9
diff --git a/conf/layer.conf b/conf/layer.conf
index 42b1b85..d031125 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -8,3 +8,5 @@
 BBFILE_COLLECTIONS += "arm"
 BBFILE_PATTERN_arm = "^${LAYERDIR}/"
 LAYERVERSION_arm = "1"
+
+LAYERSERIES_COMPAT_arm = "dunfell gatesgarth"