MAINTAINERS: add drivers/ptp/ptp_qoriq_debugfs.c into QorIQ PTP list

Added drivers/ptp/ptp_qoriq_debugfs.c into QorIQ PTP clock driver list.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>