target/riscv: Implement SGEIP bit in hip and hie CSRs

A hypervisor can optionally take guest external interrupts using
SGEIP bit of hip and hie 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: 20220204174700.534953-3-anup@brainfault.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>