net: stmmac: Add Toshiba Visconti SoCs glue driver

Add dwmac-visconti to the stmmac driver in Toshiba Visconti ARM SoCs.
This patch contains only the basic function of the device. There is no
clock control, PM, etc. yet. These will be added in the future.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>