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>

doc/develop/expo.rst

@@ -361,6 +361,13 @@ item-id
     Specifies the ID for each menu item. These are used for checking which item
     has been selected.
 
+item-value
+    type: u32 list, optional
+
+    Specifies the value for each menu item. These are used for saving and
+    loading. If this is omitted the value is its position in the menu (0..n-1).
+    Valid values are positive and negative integers INT_MIN...(INT_MAX - 1).
+
 item-label / item-label-id
     type: string list / u32 list, required
 
@@ -474,6 +481,9 @@ strings are provided inline in the nodes where they are used.
                     /* IDs for the menu items */
                     item-id = <ID_CPU_SPEED_1 ID_CPU_SPEED_2
                         ID_CPU_SPEED_3>;
+
+                    /* values for the menu items */
+                    item-value = <(-1) 3 6>;
                 };
 
                 power-loss {