net: phy: Introduce helper functions for time stamping support.

Some parts of the networking stack and at least one driver test fields
within the 'struct phy_device' in order to query time stamping
capabilities and to invoke time stamping methods.  This patch adds a
functional interface around the time stamping fields.  This will allow
insulating the callers from future changes to the details of the time
stamping implemenation.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>