ARM: shmobile: porter: add Ether DT support

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

This  patch  is analogous to the commit 26b0d2cf

 ("ARM: shmobile:
henninger: add Ether DT support") as there  are no differences between
those boards in this respect.

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