net: dc2114x: Add DM support

With all the changes in place, add support for DM into the
dc2114x driver.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Ramon Fried <rfried.dev@gmail.com>