KVM: RISC-V: Add KVM_REG_RISCV for ONE_REG interface

We will be using ONE_REG interface accessing VCPU registers from
user-space hence we add KVM_REG_RISCV for RISC-V VCPU registers.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alexander Graf <graf@amazon.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>