net: ethernet: ti: netcp_ethss: add support for child cpts node

Allow to place CPTS properties in the child "cpts" DT node. For backward
compatibility - roll-back and read CPTS DT properties from parent node if
"cpts" node is not present.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>