ARM: dts: wheat: add Ethernet support

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

Based  on the original (and large) patch by Vladimir Barinov
<vladimir.barinov@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>