spl: fit: Also record architecture in /fit-images

On ARM64 secure OS can run as 64bit or 32bit that's why it is necessary to
record information about architecture that other code can read it and
properly pass it to TF-A and start in 64bit or 32bit mode.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>