dt-bindings: serial: sh-sci: Document r8a774b1 bindings

RZ/G2N (R8A774B1) SoC also has the R-Car Gen3 compatible SCIF and
HSCIF ports, so document the SoC specific bindings.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1568724324-26995-1-git-send-email-biju.das@bp.renesas.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>