Fix example FIT image source files

The example FIT image source files do not compile with the latest dtc and
mkimage. The following error message is produced:

DTC: dts->dtb  on file "kernel.its"
Error: kernel.its 7:0 - 1:0 syntax error
FATAL ERROR: Unable to parse input tree
./mkimage: Can't read kernel.itb.tmp: Invalid argument

The FIT image source files are missing the "/dts-v1/;" directive at the
beginning of the file. Add the directive to the examples.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
diff --git a/doc/uImage.FIT/update3.its b/doc/uImage.FIT/update3.its
index 285cf73..a6eaef6 100644
--- a/doc/uImage.FIT/update3.its
+++ b/doc/uImage.FIT/update3.its
@@ -1,6 +1,9 @@
 /*
  * Example Automatic software update file.
  */
+
+/dts-v1/;
+
 / {
 	description = "Automatic software updates: kernel, ramdisk, FDT";
 	#address-cells = <1>;