ARM64: zynqmp: Generate handoff structure for ATF

ARM64: zynqmp: Generate handoff structure for ATF

Xilinx ATF extending options for passing images from BL2(FSBL)
to BL31. U-Boot SPL is FSBL replacement that's why it should generate
handoff structure the same. Support only one entry which is U-Boot in
EL2 itself. When FIT image is adopted structure generate should be data
driven.

Currently ATF is placing this structure at the beggining of OCM which is
rewriting early parts of ATF which should be unused at that time.

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