meta-intel: 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: I2d850e47d6faa8c0d4ed33fc62167aaf782d7492
diff --git a/conf/layer.conf b/conf/layer.conf
index 4ae19c7..e515b5f 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -8,5 +8,5 @@
 BBFILE_COLLECTIONS += "intel"
 BBFILE_PATTERN_intel = "^${LAYERDIR}/"
 LAYERVERSION_intel = "1"
-LAYERSERIES_COMPAT_intel = "dunfell"
+LAYERSERIES_COMPAT_intel = "dunfell gatesgarth"
 LAYERDEPENDS_intel = "phosphor-layer"