expo: Support menu-item values in cedit

Update the cedit read/write functions to support menu items with
values.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/boot/files/expo_layout.dts b/test/boot/files/expo_layout.dts
index ebe5adb..9bc1e49 100644
--- a/test/boot/files/expo_layout.dts
+++ b/test/boot/files/expo_layout.dts
@@ -40,10 +40,10 @@
 					ID_CPU_SPEED_3>;
 
 				/* values for the menu items */
-				item-value = <(-1) 3 6>;
+				item-value = <0 3 6>;
 
 				start-bit = <0x400>;
-				bit-length = <2>;
+				bit-length = <3>;
 			};
 
 			power-loss {