net: sh_eth: Add r8a7794 support

Signed-off-by: Hisashi Nakamura <hisashi.nakamura.ak@renesas.com>
[uli: added bindings documentation]
Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>