ARM: shmobile: silk: add Ether DT support

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

Based on the original patch by Vladimir Barinov
<vladimir.barinov@cogentembedded.com>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>