ice: add ice driver PTP pin documentation

Add a description of PTP pins support by the adapters to ice driver
documentation.

Reviewed-by: Milena Olech <milena.olech@intel.com>
Signed-off-by: Karol Kolacinski <karol.kolacinski@intel.com>
Signed-off-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Tested-by: Rinitha S <sx.rinitha@intel.com> (A Contingent worker at Intel)
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

Documentation/networking/device_drivers/ethernet/intel/ice.rst

@@ -927,6 +927,19 @@ To enable/disable UDP Segmentation Offload, issue the following command::
 
   # ethtool -K <ethX> tx-udp-segmentation [off|on]
 
+PTP pin interface
+-----------------
+All adapters support standard PTP pin interface. SDPs (Software Definable Pin)
+are single ended pins with both periodic output and external timestamp
+supported. There are also specific differential input/output pins (TIME_SYNC,
+1PPS) with only one of the functions supported.
+
+There are adapters with DPLL, where pins are connected to the DPLL instead of
+being exposed on the board. You have to be aware that in those configurations,
+only SDP pins are exposed and each pin has its own fixed direction.
+To see input signal on those PTP pins, you need to configure DPLL properly.
+Output signal is only visible on DPLL and to send it to the board SMA/U.FL pins,
+DPLL output pins have to be manually configured.
 
 GNSS module
 -----------