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>
diff --git a/drivers/core/Makefile b/drivers/core/Makefile
index 5edd4e4..3742e75 100644
--- a/drivers/core/Makefile
+++ b/drivers/core/Makefile
@@ -2,7 +2,7 @@
 #
 # Copyright (c) 2013 Google, Inc
 
-obj-y	+= device.o fdtaddr.o lists.o root.o uclass.o util.o
+obj-y	+= device.o fdtaddr.o lists.o root.o uclass.o util.o tag.o
 obj-$(CONFIG_$(SPL_TPL_)ACPIGEN) += acpi.o
 obj-$(CONFIG_DEVRES) += devres.o
 obj-$(CONFIG_$(SPL_)DM_DEVICE_REMOVE)	+= device-remove.o