net: ethernet: ti: cpts: use devm_get_clk_from_child

Use devm_get_clk_from_child() instead of devm_clk_get() and this way allow
to group CPTS DT properties in sub-node for better code readability and
maintenance. Roll-back to devm_clk_get() if devm_get_clk_from_child()
fails for backward compatibility.

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>