upl: Add basic tests

Add some unit tests to check that we can write a UPL handoff and read it
back.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 04a6f23..acb92bd 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1715,6 +1715,7 @@
 T:	git https://source.denx.de/u-boot/custodians/u-boot-dm.git
 F:	boot/upl*
 F:	include/upl.h
+F:	test/boot/upl.c
 
 USB
 M:	Marek Vasut <marex@denx.de>