net: add Socionext AVE ethernet driver support

Add driver for Socionext AVE ethernet controller that includes MAC and
MDIO bus supporting RGMII/RMII modes.
The driver behaves the ethernet driver model (DM_ETH) with devicetree.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>