net: ethernet: ti: allow cpts to be built separately

TI CPTS IP is used as part of TI OMAP CPSW driver, but it's also
present as part of NETCP on TI Keystone 2 SoCs. So, It's required
to enable build of CPTS for both this drivers and this can be
achieved by allowing CPTS to be built separately.

Hence, allow cpts to be built separately and convert it to be
a module as both CPSW and NETCP drives can be built as modules.

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