Document Gigabit Ethernet IP found on RZ/G2L SoC.

Gigabit Ethernet Interface includes Ethernet controller (E-MAC),
Internal TCP/IP Offload Engine (TOE) and Dedicated Direct memory
access controller (DMAC) for transferring transmitted Ethernet
frames to and received Ethernet frames from respective storage
areas in the URAM at high speed.
Signed-off-by: NBiju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: NLad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20210727123450.15918-1-biju.das.jz@bp.renesas.comSigned-off-by: NRob Herring <robh@kernel.org>