ARM: dts: sk-rzg1e: add Ether support

Define the SK-RZG1E board dependent part of the Ether device node.
Enable DHCP and NFS root  for the kernel booting.

Based on the original (and large) patch by Dmitry Shifrin
<dmitry.shifrin@cogentembedded.com>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>