ptp: ocp: Expose various resources on the timecard.

The OpenCompute timecard driver has additional functionality besides
a clock.  Make the following resources available:

 - The external timestamp channels (ts0/ts1)
 - devlink support for flashing and health reporting
 - GPS and MAC serial ports
 - board serial number (obtained from i2c device)

Also add watchdog functionality for when GNSS goes into holdover.

The resources are collected under a timecard class directory:

  [jlemon@timecard ~]$ ls -g /sys/class/timecard/ocp1/
  total 0
  -r--r--r--. 1 root 4096 Aug  3 19:49 available_clock_sources
  -rw-r--r--. 1 root 4096 Aug  3 19:49 clock_source
  lrwxrwxrwx. 1 root    0 Aug  3 19:49 device -> ../../../0000:04:00.0/
  -r--r--r--. 1 root 4096 Aug  3 19:49 gps_sync
  lrwxrwxrwx. 1 root    0 Aug  3 19:49 i2c -> ../../xiic-i2c.1024/i2c-2/
  drwxr-xr-x. 2 root    0 Aug  3 19:49 power/
  lrwxrwxrwx. 1 root    0 Aug  3 19:49 pps ->
  ../../../../../virtual/pps/pps1/
  lrwxrwxrwx. 1 root    0 Aug  3 19:49 ptp -> ../../ptp/ptp2/
  -r--r--r--. 1 root 4096 Aug  3 19:49 serialnum
  lrwxrwxrwx. 1 root    0 Aug  3 19:49 subsystem ->
  ../../../../../../class/timecard/
  lrwxrwxrwx. 1 root    0 Aug  3 19:49 ttyGPS -> ../../tty/ttyS7/
  lrwxrwxrwx. 1 root    0 Aug  3 19:49 ttyMAC -> ../../tty/ttyS8/
  -rw-r--r--. 1 root 4096 Aug  3 19:39 uevent

The labeling is needed at the minimum, in order to tell the serial
devices apart.

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