SPL ATF support requires to have firmware property which should be also
listed by mkimage -l when images is created.
The patch is also using this macro in spl_fit to match keyword.
When image is created:
Default Configuration: 'config'
Configuration 0 (config)
Description: ATF with full u-boot
Kernel: unavailable
Firmware: atf
FDT: dtb
Signed-off-by: Michal Simek <michal.simek@xilinx.com>