Fix example FIT image source files

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>