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>
diff --git a/MAINTAINERS b/MAINTAINERS
index dd099d6..78ec857 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -696,9 +696,11 @@
 BOOTDEVICE
 M:	Simon Glass <sjg@chromium.org>
 S:	Maintained
+F:	boot/bootdev*.c
 F:	boot/bootstd.c
-F:	include/bootstd.h
+F:	include/bootdev*.h
 F:	include/bootflow.h
+F:	include/bootstd.h
 
 BTRFS
 M:	Marek Behun <marek.behun@nic.cz>