target/riscv: Implement AIA mtopi, stopi, and vstopi CSRs

The AIA specification introduces new [m|s|vs]topi CSRs for
reporting pending local IRQ number and associated IRQ priority.

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: 20220204174700.534953-14-anup@brainfault.org
[ Changed by AF:
 - Fixup indentation
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>