RISC-V: KVM: Skeletal in-kernel AIA irqchip support

To incrementally implement in-kernel AIA irqchip support, we first
add minimal skeletal support which only compiles but does not provide
any functionality.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>