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>

hw/riscv/sifive_u.c

@@ -118,6 +118,7 @@ static void create_fdt(SiFiveUState *s, const MemMapEntry *memmap,
             error_report("load_device_tree() failed");
             exit(1);
         }
+        return;
     } else {
         fdt = ms->fdt = create_device_tree(&fdt_size);
         if (!fdt) {

hw/riscv/virt.c

@@ -1015,6 +1015,7 @@ static void create_fdt(RISCVVirtState *s, const MemMapEntry *memmap)
             error_report("load_device_tree() failed");
             exit(1);
         }
+        return;
     } else {
         ms->fdt = create_device_tree(&s->fdt_size);
         if (!ms->fdt) {