bootstd: doc: Add documentation

Add documentation for this feature, including the commands and full
devicetree bindings.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 963ec91..bda7b81 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -702,6 +702,10 @@
 F:	boot/bootstd.c
 F:	cmd/bootdev.c
 F:	cmd/bootflow.c
+F:	doc/develop/bootstd.rst
+F:	doc/usage/bootdev.rst
+F:	doc/usage/bootflow.rst
+F:	doc/usage/bootmeth.rst
 F:	drivers/mmc/mmc_bootdev.c
 F:	include/bootdev.h
 F:	include/bootflow.h