dt-bindings: serial: sh-sci: Document r8a774a1 bindings

dt-bindings: serial: sh-sci: Document r8a774a1 bindings

RZ/G2M (R8A774A1) SoC also has the R-Car Gen3 compatible SCIF and
HSCIF ports, so document the SoC specific bindings. While at it,
update the RZ/G1 and RZ/G2 family specific strings description as
outdated.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>