net: dc2114x: Introduce private data

Introduce dc2114x_priv, which is a super-structure around eth_device
and tracks per-device state and the device IO address.

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