xilinx: zynqmp: Fix SPL_FS_LOAD_PAYLOAD_NAME usage

SPL_FS_LOAD_PAYLOAD_NAME depends on SPL to be enabled.
If SPL is not enabled code still expects SPL_FS_LOAD_PAYLOAD_NAME to be
present. That's why setup proper dependency in the code.
And by doing so also change the logic around dfu_alt_info string
composition to be simpler.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/3989c390a4acae13a1b05c040e14fb3d68bced02.1669986373.git.michal.simek@amd.com