hw/intc: sifive_plic: Convert the PLIC to use RISC-V CPU GPIO lines

hw/intc: sifive_plic: Convert the PLIC to use RISC-V CPU GPIO lines

Instead of using riscv_cpu_update_mip() let's instead use the new RISC-V
CPU GPIO lines to set the external MIP bits.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 0364190bfa935058a845c0fa1ecf650328840ad5.1630301632.git.alistair.francis@wdc.com