tg3: Enable support for timesync gpio output

The PTP_CAPABLE tg3 devices have a gpio output that is toggled when the
free running counter matches a watchdog value. This patch adds support
to set the watchdog and enable this feature.

Since the output is controlled via bits in the EAV_REF_CLCK_CTL
register, we have to read-modify-write it when we stop/resume.

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>