hw/riscv/virt.c: simplify create_fdt()

hw/riscv/virt.c: simplify create_fdt()

'mem_size' and 'cmdline' aren't being used. Remove them.

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>
Message-Id: <20230111170948.316276-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>