hw/riscv: Allow creating multiple instances of PLIC

hw/riscv: Allow creating multiple instances of PLIC

We extend PLIC emulation to allow multiple instances of PLIC in
a QEMU RISC-V machine. To achieve this, we remove first HART id
zero assumption from PLIC emulation.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20200616032229.766089-3-anup.patel@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>