xilinx: zynq: Generate dfu for SPI only when enabled

Bitmain S9 board is not enabling SPI that's why CONFIG_SYS_SPI_U_BOOT_OFFS
symbol is not present which ends in build failure when EFI/DFU
infrastructure is enabled.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/3ea0572dbda8f21270c00a77528eb6ec954c7c40.1699866313.git.michal.simek@amd.com