hw/riscv: sifive_u: Use the PLIC config helper function

hw/riscv: sifive_u: Use the PLIC config helper function

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 20211022060133.3045020-3-alistair.francis@opensource.wdc.com