sh_eth: add R8A77980 support

Finally, add support for the DT probing of the R-Car V3H (AKA R8A77980) --
it's the only R-Car gen3 SoC having the GEther controller -- others have
only EtherAVB...

Based on the original (and large) patch by Vladimir Barinov.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>