bootstd: Add the bootstd uclass and core implementation

The 'bootstd' device provides the central information about U-Boot
standard boot.

Add a uclass for bootstd and the various helpers needed to make it
work. Also add a binding file.

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_BOOTSTD,		/* Standard boot driver */
 	UCLASS_BUTTON,		/* Button */
 	UCLASS_CACHE,		/* Cache controller */
 	UCLASS_CLK,		/* Clock source, e.g. used by peripherals */