net/ethoc: add CONFIG_DM_ETH support

Extract reusable parts from ethoc_init, ethoc_set_mac_address,
ethoc_send and ethoc_receive, move the rest under #ifdef CONFIG_DM_ETH.
Add U_BOOT_DRIVER, eth_ops structure and implement required methods.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>