dm: blk: Add a block-device uclass

Add a uclass for block devices. These provide block-oriented data access,
supporting reading, writing and erasing of whole blocks.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
diff --git a/include/dm/uclass-id.h b/include/dm/uclass-id.h
index 3bea308..37c4176 100644
--- a/include/dm/uclass-id.h
+++ b/include/dm/uclass-id.h
@@ -26,6 +26,7 @@
 
 	/* U-Boot uclasses start here - in alphabetical order */
 	UCLASS_ADC,		/* Analog-to-digital converter */
+	UCLASS_BLK,		/* Block device */
 	UCLASS_CLK,		/* Clock source, e.g. used by peripherals */
 	UCLASS_CPU,		/* CPU, typically part of an SoC */
 	UCLASS_CROS_EC,		/* Chrome OS EC */