hw/riscv: virt: Move flash node to root

hw/riscv: virt: Move flash node to root

The flash is not inside the SoC, so it's inappropriate to put it
under the /soc node. Move it to root instead.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210807035641.22449-1-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>