dt-bindings: serial: sh-sci: Document r8a7744 bindings

RZ/G1N (R8A7744) SoC also has the R-Car gen2 compatible SCIF, SCIFA,
SCIFB, and HSCIF ports, so document the SoC specific bindings.

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