Add support for driver model, so that CONFIG_DM_ETH can be defined and
used with this driver.

This patch also adds the read_rom_hwaddr() callback so that the ROM MAC
address will be used to the DM part of this driver.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Ted Chen <tedchen@realtek.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Joe Hershberger <joe.hershberger@ni.com>