ptp: ocp: Add second GNSS device

Upcoming boards may have a second GNSS receiver, getting information
from a different constellation than the first receiver, which provides
some measure of anti-spoofing.

Expose the sysfs attribute for this device, if detected.

Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>