ethtool: Introduce a method for getting time stamping capabilities.

This commit adds a new ethtool ioctl that exposes the SO_TIMESTAMPING
capabilities of a network interface. In addition, user space programs
can use this ioctl to discover the PTP Hardware Clock (PHC) device
associated with the interface.

Since software receive time stamps are handled by the stack, the generic
ethtool code can answer the query correctly in case the MAC or PHY
drivers lack special time stamping features.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>