dt-bindings: serial: renesas, hscif: Document r8a774e1 bindings

RZ/G2H (R8A774E1) SoC also has the R-Car gen3 compatible HSCIF ports,
so document the SoC specific bindings.

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>
Link: https://lore.kernel.org/r/1594230511-24790-3-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com


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