arm: socfpga: soc64: Load FIT image with ATF support

Instead of loading u-boot proper image (u-boot.img), SPL
now loads FIT image (u-boot.itb) which includes u-boot
proper, ATF and u-boot proper's DTB.

For OS, u-boot now loads FIT images (kernel.itb) which
includes Linux Image and Linux's DTB.

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>
Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>