target/riscv: Add AIA cpu feature

We define a CPU feature for AIA CSR support in RISC-V CPUs which
can be set by machine/device emulation. The RISC-V CSR emulation
will also check this feature for emulating AIA CSRs.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-id: 20220204174700.534953-7-anup@brainfault.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>