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/ahci-uclass.c b/drivers/block/ahci-uclass.c
new file mode 100644
index 0000000..7b8c326
--- /dev/null
+++ b/drivers/block/ahci-uclass.c
@@ -0,0 +1,14 @@
+/*
+ * Copyright (c) 2015 Google, Inc
+ * Written by Simon Glass <sjg@chromium.org>
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#include <common.h>
+#include <dm.h>
+
+UCLASS_DRIVER(ahci) = {
+	.id		= UCLASS_AHCI,
+	.name		= "ahci",
+};