net: mscc: PTP Hardware Clock (PHC) support

This patch adds support for PTP Hardware Clock (PHC) to the Ocelot
switch for both PTP 1-step and 2-step modes.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>