target/riscv: Implement AIA IMSIC interface CSRs

The AIA specification defines IMSIC interface CSRs for easy access
to the per-HART IMSIC registers without using indirect xiselect and
xireg CSRs. This patch implements the AIA IMSIC interface CSRs.

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: 20220204174700.534953-16-anup@brainfault.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>