target/riscv: Implement AIA interrupt filtering CSRs

The AIA specificaiton adds interrupt filtering support for M-mode
and HS-mode. Using AIA interrupt filtering M-mode and H-mode can
take local interrupt 13 or above and selectively inject same local
interrupt to lower privilege modes.

At the moment, we don't have any local interrupts above 12 so we
add dummy implementation (i.e. read zero and ignore write) of AIA
interrupt filtering CSRs.

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: 20220204174700.534953-13-anup@brainfault.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>