xilinx: zynqmp: Use CONFIG_SPL_FS_LOAD_PAYLOAD_NAME

There is no need to use u-boot.itb as name when this is already recorded in
CONFIG_SPL_FS_LOAD_PAYLOAD_NAME macro.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Link: https://lore.kernel.org/r/38a236ed317510c26b37525da0e7bc26b411222c.1634558534.git.michal.simek@xilinx.com