riscv/sifive_u: fix a memory leak in soc_realize()

riscv/sifive_u: fix a memory leak in soc_realize()

Fix a minor memory leak in riscv_sifive_u_soc_realize()

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>