hw/intc: Remove the Ibex PLIC

The Ibex PLIC is now spec compliant. Let's remove the Ibex PLIC and
instead use the SiFive PLIC.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 5557935c2660c5e6281b6d21e6514e019593662e.1634524691.git.alistair.francis@wdc.com