net: ethernet: ti: am65-cpts: add pps support

CPTS doesn't have HW support for PPS ("pulse per second”) signal
generation, but it can be modeled by using Time Sync Router and routing
GenFx (periodic signal generator) output to CPTS_HWy_TS_PUSH (hardware time
stamp) input, and configuring GenFx to generate 1sec pulses.

     +------------------------+
     |          CPTS          |
     |                        |
 +--->CPTS_HW4_PUSH      GENFx+---+
 |   |                        |   |
 |   +------------------------+   |
 |                                |
 +--------------------------------+

Add corresponding support to am65-cpts driver. The DT property "ti,pps"
has to be used to enable PPS support and configure pair
[CPTS_HWy_TS_PUSH, GenFx].

Once enabled, PPS can be tested using ppstest tool:
 # ./ppstest /dev/pps0

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>