fec: Restart PPS after link state change

On link state change, the controller gets reset,
causing PPS to drop out and the PHC to lose its
time and calibration. So we restart it if needed,
restoring calibration and time registers.

Changes since v2:
* Add `fec_ptp_save_state()`/`fec_ptp_restore_state()`
* Use `ktime_get_real_ns()`
* Use `BIT()` macro
Changes since v1:
* More ECR #define's
* Stop PPS in `fec_ptp_stop()`

Signed-off-by: Csókás Bence <csokas.bence@prolan.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>