target/riscv: Implement hgeie and hgeip CSRs

target/riscv: Implement hgeie and hgeip CSRs

The hgeie and hgeip CSRs are required for emulating an external
interrupt controller capable of injecting virtual external interrupt
to Guest/VM running at VS-level.

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: 20220204174700.534953-4-anup@brainfault.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>