ptp: Enable clock drivers along with associated net/PHY drivers

Where a PTP clock driver is associated with a net or PHY driver, it
should be enabled automatically whenever that driver is enabled.
Therefore:

- Make PTP clock drivers select rather than depending on PTP_1588_CLOCK
- Remove separate boolean options for PTP clock drivers that are built
  as part of net driver modules.  (This also fixes cases where the PTP
  subsystem is wrongly forced to be built-in.)
- Set 'default y' for PTP clock drivers that depend on specific net
  drivers but are built separately

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