KVM: arm64: Document HVC_VHE_RESTART stub hypercall

For completeness, let's document the HVC_VHE_RESTART stub.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: David Brazdil <dbrazdil@google.com>
Link: https://lore.kernel.org/r/20210208095732.3267263-19-maz@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>