arm64: zynqmp: Add support for u-boot.itb generation with ATF

arm64: zynqmp: Add support for u-boot.itb generation with ATF

Follow i.MX, Sunxi, RISC-V and Rockchip to generate u-boot.itb which
includes U-Boot proper, ATF and DTBs in FIT format. ZynqMP supports FIT for
quite a long time but with using out of tree solution. The patch is filling
this gap.

Tested on zcu102, zcu104 and zcu100/Ultra96.

zcu100/Ultra96 v2.2 ATF build by:
make DEBUG=0 ZYNQMP_CONSOLE=cadence1 RESET_TO_BL31=1 PLAT=zynqmp bl31

Signed-off-by: Michal Simek <michal.simek@xilinx.com>