upl: Add support for Universal Payload in SPL

Add the basic code to create a handoff structure in SPL, so it can be
passed to the next phase. For now this is not plumbed in.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 4d17c96..43d430c 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:	cmd/upl.c
+F:	common/spl/spl_upl.c
 F:	doc/usage/cmd/upl.rst
 F:	include/upl.h
 F:	test/boot/upl.c