dm: Rename disk uclass to ahci

This started as 'ahci' and was renamed to 'disk' during code review. But it
seems that this is too generic. Now that we have a 'blk' uclass, we can use
that as the generic piece, and revert to ahci for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
index fcc9ccd..80eea84 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -9,10 +9,9 @@
 	  be partitioned into several areas, called 'partitions' in U-Boot.
 	  A filesystem can be placed in each partition.
 
-config DISK
-	bool "Support disk controllers with driver model"
+config AHCI
+	bool "Support SATA controllers with driver model"
 	depends on DM
-	default y if DM
 	help
 	  This enables a uclass for disk controllers in U-Boot. Various driver
 	  types can use this, such as AHCI/SATA. It does not provide any standard