tools: zynqmpimage: Add Xilinx ZynqMP boot header generation

Add support for the zynqmpimage to mkimage.
Only basic functionality is supported without encryption and register
initialization with one partition which is filled by U-Boot SPL.
For more detail information look at Xilinx ZynqMP TRM.

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