dt-bindings: serial: renesas,scifa: Document r8a7742 bindings

RZ/G1H (R8A7742) SoC also has the R-Car gen2 compatible SCIFA ports,
so document the SoC specific bindings.

Reviewed-by: Marian-Cristian Rotariu <marian-cristian.rotariu.rb@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Rob Herring <robh@kernel.org>