expo: Support building an expo from a description file

The only way to create an expo at present is by calling the functions to
create each object. It is useful to have more data-driven approach, where
the objects can be specified in a suitable file format and created from
that. This makes testing easier as well.

Add support for describing an expo in a devicetree node. This allows more
complex tests to be set up, as well as providing an easier format for
users. It also provides a better basis for the upcoming configuration
editor.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 38d5739..442222e 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -141,6 +141,9 @@
 		};
 	};
 
+	cedit: cedit {
+	};
+
 	fuzzing-engine {
 		compatible = "sandbox,fuzzing-engine";
 	};
@@ -1830,3 +1833,5 @@
 #ifdef CONFIG_SANDBOX_VPL
 #include "sandbox_vpl.dtsi"
 #endif
+
+#include "cedit.dtsi"