ptp_qoriq: fix NULL access if ptp dt node missing

Make sure ptp dt node exists before accessing it in case
of NULL pointer call trace.

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>