meta-mellanox: 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: I60ffc105ef10a562c2aa4009fc5b3ff939209f40
diff --git a/conf/layer.conf b/conf/layer.conf
index 9b1c6b9..8e43822 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -3,3 +3,5 @@
 
 BBFILE_COLLECTIONS += "mellanox"
 BBFILE_PATTERN_mellanox = ""
+
+LAYERSERIES_COMPAT_mellanox = "dunfell gatesgarth"