RISC-V: Add hypervisor extension related CSR defines

This patch adds asm/kvm_csr.h for RISC-V hypervisor extension
related defines.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alexander Graf <graf@amazon.com>
Message-Id: <20210927114016.1089328-2-anup.patel@wdc.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>