net: mvpp2: tai: add extts support

[Shmuel Hazan]

This patch series adds support for PTP event capture on the Aramda
80x0/70x0. This feature is mainly used by tools linux ts2phc(3) in order
to synchronize a timestamping unit (like the mvpp2's TAI) and a system
DPLL on the same PCB. 

The patch series includes 3 patches: the second one implements the
actual extts function.