hw/intc: Add RISC-V AIA IMSIC device emulation

hw/intc: Add RISC-V AIA IMSIC device emulation

The RISC-V AIA (Advanced Interrupt Architecture) defines a new
interrupt controller for MSIs (message signal interrupts) called
IMSIC (Incoming Message Signal Interrupt Controller). The IMSIC
is per-HART device and also suppport virtualizaiton of MSIs using
dedicated VS-level guest interrupt files.

This patch adds device emulation for RISC-V AIA IMSIC which
supports M-level, S-level, and VS-level MSIs.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-Id: <20220220085526.808674-3-anup@brainfault.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>