ptp: Add adjphase function to support phase offset control.

Adds adjust phase function to take advantage of a PHC
clock's hardware filtering capability that uses phase offset
control word instead of frequency offset control word.

Signed-off-by: Vincent Cheng <vincent.cheng.xh@renesas.com>
Reviewed-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>