hw/riscv: sifive_u: Use SIFIVE_U_CPU for mc->default_cpu_type

hw/riscv: sifive_u: Use SIFIVE_U_CPU for mc->default_cpu_type

SIFIVE_U_CPU is conditionally set to SIFIVE_U34 or SIFIVE_U54, hence
there is no need to use #idef to set the mc->default_cpu_type.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210109143637.29645-1-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>