treewide: remove gategarth from layer-support

We've typically kept these LAYERSERIES_COMPAT to 2 releases: the current
and the upcoming.  Remove 'gatesgarth' is it is now 2 releases back.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5e812a94fed1738898af75c0fdee81996a5bbf20
diff --git a/meta-openpower/conf/layer.conf b/meta-openpower/conf/layer.conf
index 4236ab2..9a16ca0 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 = "gatesgarth hardknott honister"
+LAYERSERIES_COMPAT_openpower-layer = "hardknott honister"