target/riscv: Implement AIA xiselect and xireg CSRs

The AIA specification defines [m|s|vs]iselect and [m|s|vs]ireg CSRs
which allow indirect access to interrupt priority arrays and per-HART
IMSIC registers. This patch implements AIA xiselect and xireg 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: 20220204174700.534953-15-anup@brainfault.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>