netdev.h should not be included in driver-model enabled builds (doing so
causes compiler warnings about struct eth_driver not being declared), but
we do use sunxi_gmac_initialize in the driver-model case, so move it out of
netdev.h .

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Ian Campbell <ijc@hellion.org.uk>