ahci: Add a disk-controller uclass

Add a uclass ID for a disk controller. This can be used by AHCI/SATA or
other controller types. There are no operations and no interface so far,
but it is possible to probe a SATA device.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
index e69de29..990f768 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -0,0 +1,9 @@
+config DISK
+	bool "Support disk 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
+	  operations at present. The block device interface has not been converted
+	  to driver model.