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/sandbox.dtsi b/arch/sandbox/dts/sandbox.dtsi
index 8aaf911..ff7e558 100644
--- a/arch/sandbox/dts/sandbox.dtsi
+++ b/arch/sandbox/dts/sandbox.dtsi
@@ -16,12 +16,6 @@
 		stdout-path = "/serial";
 	};
 
-	cedit-theme {
-		font-size = <30>;
-		menu-inset = <3>;
-		menuitem-gap-y = <1>;
-	};
-
 	alarm_wdt: alarm-wdt {
 		compatible = "sandbox,alarm-wdt";
 		timeout-sec = <5>;
@@ -36,6 +30,12 @@
 	bootstd {
 		compatible = "u-boot,boot-std";
 		filename-prefixes = "./";
+
+		cedit-theme {
+			font-size = <30>;
+			menu-inset = <3>;
+			menuitem-gap-y = <1>;
+		};
 	};
 
 	buttons {