bootstd: Add the bootdev uclass

A 'bootdev' is a device which can be used to boot an operating system.
It is a child of the media device (e.g. MMC) which handles reading files
from that device, such as a bootflow file.

Add a uclass for bootdev and the various helpers needed to make it
work. Also add a binding file, empty for now.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/dm/uclass-id.h

@@ -38,6 +38,7 @@ enum uclass_id {
 	UCLASS_AXI,		/* AXI bus */
 	UCLASS_BLK,		/* Block device */
 	UCLASS_BOOTCOUNT,       /* Bootcount backing store */
+	UCLASS_BOOTDEV,		/* Boot device for locating an OS to boot */
 	UCLASS_BOOTSTD,		/* Standard boot driver */
 	UCLASS_BUTTON,		/* Button */
 	UCLASS_CACHE,		/* Cache controller */