drivers: net: keystone_net: drop non dm code

drivers: net: keystone_net: drop non dm code

Networking support for all TI K2 boards converted to use DM model and
CONFIG_DM_ETH enabled in all corresponding defconfig files, hence drop
unused non DM K2 networking code.

Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>