hw/riscv: virt: Re-factor FDT generation

hw/riscv: virt: Re-factor FDT generation

We re-factor and break the FDT generation into smaller functions
so that it is easier to modify FDT generation for different
configurations of virt machine.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 20210831110603.338681-4-anup.patel@wdc.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>