upl: Add a command

Add a 'upl' command to work with Universal Payload features. For now it
only supports reading and writing a handoff structure.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index acb92bd..4d17c96 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1714,6 +1714,8 @@
 S:	Maintained
 T:	git https://source.denx.de/u-boot/custodians/u-boot-dm.git
 F:	boot/upl*
+F:	cmd/upl.c
+F:	doc/usage/cmd/upl.rst
 F:	include/upl.h
 F:	test/boot/upl.c