expo: Move cedit theme under bootstd

This is related to standard boot, so put it under the same node. This may
simplify schema upstreaming later.

Mention themes in the documentation while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index f351d5c..084cb88 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -100,6 +100,12 @@
 			menuitem-gap-y = <1>;
 		};
 
+		cedit-theme {
+			font-size = <30>;
+			menu-inset = <3>;
+			menuitem-gap-y = <1>;
+		};
+
 		/*
 		 * This is used for the VBE OS-request tests. A FAT filesystem
 		 * created in a partition with the VBE information appearing
@@ -144,12 +150,6 @@
 	cedit: cedit {
 	};
 
-	cedit-theme {
-		font-size = <30>;
-		menu-inset = <3>;
-		menuitem-gap-y = <1>;
-	};
-
 	fuzzing-engine {
 		compatible = "sandbox,fuzzing-engine";
 	};