riscv: Add PLIC device emulation

The PLIC (platform level interrupt controller) manages peripheral
interrupts in RISC-V world. The per-CPU interrupts are managed
using CPU CSRs hence virtualized in-kernel by KVM RISC-V.

This patch adds PLIC device emulation for KVMTOOL RISC-V.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
[For PLIC context CLAIM register emulation]
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Link: https://lore.kernel.org/r/20211119124515.89439-6-anup.patel@wdc.com


Signed-off-by: Will Deacon <will@kernel.org>