hw/riscv: Skip re-generating DT nodes for a given DTB

hw/riscv: Skip re-generating DT nodes for a given DTB

Launch qemu-system-riscv64 with a given dtb for 'sifive_u' and 'virt'
machines, QEMU complains:

  qemu_fdt_add_subnode: Failed to create subnode /soc: FDT_ERR_EXISTS

The whole DT generation logic should be skipped when a given DTB is
present.

Fixes: b1f19f238cae ("hw/riscv: write bootargs 'chosen' FDT after riscv_load_kernel()")
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20230228074522.1845007-1-bmeng@tinylab.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>