hw/riscv: Move the dtb load bits outside of create_fdt()

hw/riscv: Move the dtb load bits outside of create_fdt()

Move the dtb load bits outside of create_fdt(), and put it explicitly
in sifive_u_machine_init() and virt_machine_init(). With such change
create_fdt() does exactly what its function name tells us.

Suggested-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20230228074522.1845007-2-bmeng@tinylab.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>