ptp: link the phc device to its parent device

PTP Hardware Clock devices appear as class devices in sysfs. This patch
changes the registration API to use the parent device, clarifying the
clock's relationship to the underlying device.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>