net: ti: icssg-prueth: Enable PTP timestamping support for SR1.0 devices

Enable PTP support for AM65x SR1.0 devices by registering with the IEP
infrastructure in order to expose a PTP clock to userspace.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: Diogo Ivo <diogo.ivo@siemens.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>