hw/riscv: set machine->fdt in sifive_u_machine_init()

hw/riscv: set machine->fdt in sifive_u_machine_init()

This will enable support for 'dumpdtb' QMP/HMP command for the sifive_u
machine.

Cc: Alistair Francis <Alistair.Francis@wdc.com>
Cc: Bin Meng <bin.meng@windriver.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220926173855.1159396-15-danielhb413@gmail.com>