dm: add tag support

With dm-tag feature, any U-Boot subsystem is allowed to associate
arbitrary number of data with a particular udevice. This can been
see as expanding "struct udevice" without modifying the definition.

As a first user, UEFI subsystem makes use of tags to associate
an efi_disk object with a block device.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/asm-generic/global_data.h

@@ -474,6 +474,10 @@ struct global_data {
 	 */
 	struct event_state event_state;
 #endif
+	/**
+	 * @dmtag_list: List of DM tags
+	 */
+	struct list_head dmtag_list;
 };
 #ifndef DO_DEPS_ONLY
 static_assert(sizeof(struct global_data) == GD_SIZE);