xilinx: zynq: Use CONFIG_SPL_FS_LOAD_PAYLOAD_NAME for dfu_alt_info

CONFIG_SPL_FS_LOAD_PAYLOAD_NAME stores the name of firmware file to be
loaded by SPL. Name can be selected via Kconfig that's why use the macro.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/601fbc2dfd16b4708fc6b5f86954e10add43334e.1660055571.git.michal.simek@amd.com