KVM: arm64: Add comments for context flush and sync callbacks

Add a little bit of information on where _ctxflush_fp() and _ctxsync_fp()
are called to help people unfamiliar with the code get up to speed.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220124155720.3943374-2-broonie@kernel.org