sh_eth: add device tree support

Add support of the device tree probing for the Renesas SH-Mobile SoCs
documenting the device tree binding as necessary.

This work is loosely based on the original patch by Nobuhiro Iwamatsu
<nobuhiro.iwamatsu.yj@renesas.com>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>