libfdt: Upgrade libfdt source files

Update the libfdt source files, the upstream commit is 73e0f143b73d
("libfdt: fdt_strerror(): Fix comparison warning").

This brings us the fixes for the signed/unsigned comparison warnings,
so platforms can enable -Wsign-compare now.

Change-Id: I303d891c82ffea0acefdde27289339db5ac5a289
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/lib/libfdt/fdt_wip.c b/lib/libfdt/fdt_wip.c
index f64139e..c2d7566 100644
--- a/lib/libfdt/fdt_wip.c
+++ b/lib/libfdt/fdt_wip.c
@@ -23,7 +23,7 @@
 	if (!propval)
 		return proplen;
 
-	if (proplen < (len + idx))
+	if ((unsigned)proplen < (len + idx))
 		return -FDT_ERR_NOSPACE;
 
 	memcpy((char *)propval + idx, val, len);