net: via-rhine: add OF bus binding

This should make the driver usable with VIA/WonderMedia ARM-based
Systems-on-Chip integrated Rhine III adapters. Note that these
are always in MMIO mode, and don't have any known EEPROM.

Signed-off-by: Alexey Charkov <alchark@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>