MAINTAINERS: add yangbolu1991 as maintainer for PTP Clock

I once worked as maintainer for Linux PTP VIRTUAL CLOCK SUPPORT
and FREESCALE QORIQ PTP CLOCK DRIVER.

Although I haven't been working on zephyr PTP for long, I have
clear understanding for current zephyr PTP clock and PTP/gPTP stack.
And started to contribute for it.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>