dt-bindings: net: Document support for Renesas RZ/V2H(P) GBETH

GBETH IP on the Renesas RZ/V2H(P) SoC is integrated with Synopsys
DesignWare MAC (version 5.20). Document the device tree bindings for
the GBETH glue layer.

Generic compatible string 'renesas,rzv2h-gbeth' is added since this
module is identical on both the RZ/V2H(P) and RZ/G3E SoCs.

The Rx/Tx clocks supplied for GBETH on the RZ/V2H(P) SoC is depicted
below:

                      Rx / Tx
-------+------------- on / off -------
       |
       |            Rx-180 / Tx-180
       +---- not ---- on / off -------

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250417084015.74154-3-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>