ptp_clock: future-proofing drivers against PTP subsystem becoming optional

Drivers must be ready to accept NULL from ptp_clock_register() if the
PTP clock subsystem is configured out.

This patch documents that and ensures that all drivers cope well
with a NULL return.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Eugenia Emantayev <eugenia@mellanox.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>