target/riscv: Allow AIA device emulation to set ireg rmw callback

The AIA device emulation (such as AIA IMSIC) should be able to set
(or provide) AIA ireg read-modify-write callback for each privilege
level of a RISC-V HART.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-id: 20220204174700.534953-9-anup@brainfault.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>