ARM: dts: blanche: add Ethernet support

R8A7792  SoC  doesn't have the EtherMAC core, so SMSC  LAN89218  Ethernet
chip was used instead on the Blanche board; this chip  is compatible with
SMSC LAN9115  for  which there's a (device tree aware) driver. Describe
the  chip in  the  Blanche device tree;  enable DHCP and NFS root in the
kernel command line for the kernel booting.

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