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>
diff --git a/MAINTAINERS b/MAINTAINERS
index d6d4557..dd099d6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -696,6 +696,8 @@
 BOOTDEVICE
 M:	Simon Glass <sjg@chromium.org>
 S:	Maintained
+F:	boot/bootstd.c
+F:	include/bootstd.h
 F:	include/bootflow.h
 
 BTRFS