dt-bindings: serial: sh-sci: Document r8a774c0 bindings

RZ/G2E (R8A774C0) SoC also has the R-Car Gen3 compatible SCIF and
HSCIF ports, so document the SoC specific bindings.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>