MAINTAINERS: add myself as maintainer for QorIQ PTP clock driver

Added myself as maintainer for QorIQ PTP clock driver.
Since gianfar_ptp.c was renamed to ptp_qoriq.c, let's
maintain it under QorIQ PTP clock driver.

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