net: dpaa2: rename rtc as ptp in dpaa2-ptp driver

In dpaa2-ptp driver, it's odd to use rtc in names of
some functions and structures except these dprtc APIs.
This patch is to use ptp instead of rtc in names.

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