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/include/dm/uclass-id.h b/include/dm/uclass-id.h
index 0e26e1d..230b1ea 100644
--- a/include/dm/uclass-id.h
+++ b/include/dm/uclass-id.h
@@ -83,6 +83,7 @@
 	UCLASS_P2SB,		/* (x86) Primary-to-Sideband Bus */
 	UCLASS_PANEL,		/* Display panel, such as an LCD */
 	UCLASS_PANEL_BACKLIGHT,	/* Backlight controller for panel */
+	UCLASS_PARTITION,	/* Logical disk partition device */
 	UCLASS_PCH,		/* x86 platform controller hub */
 	UCLASS_PCI,		/* PCI bus */
 	UCLASS_PCI_EP,		/* PCI endpoint device */