riscv: KVM: Apply insn-def to hfence encodings

Introduce hfence instruction encodings and apply them to KVM's use.
With the self-documenting nature of the instruction encoding macros,
and a spec always within arm's reach, it's safe to remove the
comments, so we do that too.

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