dm: disk: add UCLASS_PARTITION

NOTE: probably we have to update config dependencies,
in particular, SPL/TPL_PRINTF?

With this new function, UCLASS_PARTITION devices will be created as
child nodes of UCLASS_BLK device.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
diff --git a/disk/Makefile b/disk/Makefile
index 5ca10c5..ec14883 100644
--- a/disk/Makefile
+++ b/disk/Makefile
@@ -6,6 +6,9 @@
 #ccflags-y += -DET_DEBUG -DDEBUG
 
 obj-$(CONFIG_$(SPL_TPL_)PARTITIONS)  += part.o
+ifdef CONFIG_$(SPL_TPL_)BLK
+obj-$(CONFIG_$(SPL_TPL_)PARTITIONS)  += disk-uclass.o
+endif
 obj-$(CONFIG_$(SPL_TPL_)MAC_PARTITION)   += part_mac.o
 obj-$(CONFIG_$(SPL_TPL_)DOS_PARTITION)   += part_dos.o
 obj-$(CONFIG_$(SPL_TPL_)ISO_PARTITION)   += part_iso.o