ptp: ocp: Add NMEA output

The timecard can provide a NMEA-1083 ZDA (time and date) output
string on a serial port, which can be used to drive other devices.

Add the NMEA resources, and the serial port as a sysfs attribute.

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