tools: xilinx: Fix zynq/zynqmp image recognition

There is an issue to recognize zynq or zynqmp image because header
checking is just the same. That's why zynqmp images are recognized as
zynq one.
Check unused fields which are initialized to zero in zynq format
(__reserved1 0x38 and __reserved2 0x44) which are initialized for
zynqmp. This should ensure that images are properly recognized by:
./tools/mkimage -l spl/boot.bin

Also show image type as ZynqMP instead of Zynq which is confusing.

Reported-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Tested-by: Alexander Graf <agraf@suse.de>