refactor(xilinx): move the atf handoff structure

Move the ATF handoff structure from the plat_startup.c to the
header file plat_startup.h, as these can be used by the platform code.

Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@xilinx.com>
Reviewed-by: Michal Simek <michal.simek@amd.com>
Change-Id: Ifb425d444eb65fe8648952d2ff64d4e92c2b340a