hw/riscv/spike: use 'fdt' from MachineState

hw/riscv/spike: use 'fdt' from MachineState

The MachineState object provides a 'fdt' pointer that is already being
used by other RISC-V machines, and it's also used by the 'dumpdtb' QMP
command.

Remove the 'fdt' pointer from SpikeState and use MachineState::fdt
instead.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Message-Id: <20230102115241.25733-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>