linker_lists: Add a function to access a linker list entry

Once declared, you cannot access a linker_list entry since you do not have
a symbol name for it. Add llsym() macro to provide this. This avoids
searching for the symbol at run-time based on name.

An example usage is to declare a driver with U_BOOT_DRIVER(), then obtain
a pointer to that driver later.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>

include/linker_lists.h

@@ -102,6 +102,16 @@
  *   %u_boot_list_2_drivers_3
  */
 
+/**
+ * ll_sym() - Access a linker-generated array entry
+ * @_type:	Data type of the entry
+ * @_name:	Name of the entry
+ * @_list:	name of the list. Should contain only characters allowed
+ *		in a C variable name!
+ */
+#define llsym(_type, _name, _list) \
+		((_type *)&_u_boot_list_2_##_list##_2_##_name)
+
 /**
  * ll_entry_declare() - Declare linker-generated array entry
  * @_type:	Data type of the entry