ravb: Add support for RZ/G2L SoC

RZ/G2L SoC has Gigabit Ethernet IP consisting of Ethernet controller
(E-MAC), Internal TCP/IP Offload Engine (TOE) and Dedicated Direct
memory access controller (DMAC).

This patch adds compatible string for RZ/G2L and fills up the
ravb_hw_info struct. Function stubs are added which will be used by
gbeth_hw_info and will be filled incrementally.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>