expo: Allow menu items to have values

At present menu items are stored according to their sequence number in
the menu. In some cases we may want to have holes in that sequence, or
not use a sequence at all.

Add a new 'value' property for menu items. This will be used for
reading and writing, if present. If there is no 'value' property, then
the normal sequence number will be used instead.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/boot/files/expo_layout.dts

@@ -39,6 +39,9 @@
 				item-id = <ID_CPU_SPEED_1 ID_CPU_SPEED_2
 					ID_CPU_SPEED_3>;
 
+				/* values for the menu items */
+				item-value = <(-1) 3 6>;
+
 				start-bit = <0x400>;
 				bit-length = <2>;
 			};