RISC-V: KVM: Add ONE_REG interface for AIA CSRs

We implement ONE_REG interface for AIA CSRs as a separate subtype
under the CSR ONE_REG interface.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>