net: mscc: allow offloading timestamping operations to the PHY

This patch adds support for offloading timestamping operations not only
to the Ocelot switch (as already supported) but to compatible PHYs.
When both the PHY and the Ocelot switch support timestamping operations,
the PHY implementation is chosen as the timestamp will happen closer to
the medium.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>