platform: sifive: fu540: Add 32-bit specific fdt/payload addresses

For testing 32-bit SiFive specific drivers with QEMU riscv32, add
32-bit specific FW_JUMP_FDT_ADDR and FW_PAYLOAD_OFFSET.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>