net: Make DM_ETH be selected by NETDEVICE

The deadline for DM_ETH migration passed 2 years ago.  Now that
platforms which cannot be migrated have been either removed or had
drivers disabled, and platforms that needed minor help to migrate have
been forcefully migrated, we can complete the migration.

This entails select'ing DM_ETH under NETDEVICES, and then removing now
extraneous depends on lines.  In a few places, we can now either remove
options or just simplify later dependencies.

Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Ramon Fried <rfried.dev@gmail.com>