dm: clarify DM_FLAG_PROBE_AFTER_BIND behaviour

dm: clarify DM_FLAG_PROBE_AFTER_BIND behaviour

The DM_FLAG_PROBE_AFTER_BIND flag only makes sense on a per-device
basis, however recently added documentation as well as some confused
drivers imply that it might be added to a driver definition, this does
nothing.

Clarify the new documentation and expand on the comment by the
definition to point people in the right direction.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Michal Simek <michal.simek@amd.com>