dt-bindings: ata: sata_rcar: Add r8a774b1 support

Document SATA support for the RZ/G2N, no driver change required.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>