dt-bindings: serial: sh-sci: Document r8a774b1 bindings

RZ/G2N (R8A774B1) SoC also has the R-Car Gen3 compatible SCIF and
HSCIF ports, so document the SoC specific bindings.
Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1568724324-26995-1-git-send-email-biju.das@bp.renesas.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/renesas,sci-serial.txt

@@ -26,6 +26,8 @@ Required properties:
     - "renesas,hscif-r8a77470" for R8A77470 (RZ/G1C) HSCIF compatible UART.
     - "renesas,scif-r8a774a1" for R8A774A1 (RZ/G2M) SCIF compatible UART.
     - "renesas,hscif-r8a774a1" for R8A774A1 (RZ/G2M) HSCIF compatible UART.
+    - "renesas,scif-r8a774b1" for R8A774B1 (RZ/G2N) SCIF compatible UART.
+    - "renesas,hscif-r8a774b1" for R8A774B1 (RZ/G2N) HSCIF compatible UART.
     - "renesas,scif-r8a774c0" for R8A774C0 (RZ/G2E) SCIF compatible UART.
     - "renesas,hscif-r8a774c0" for R8A774C0 (RZ/G2E) HSCIF compatible UART.
     - "renesas,scif-r8a7778" for R8A7778 (R-Car M1) SCIF compatible UART.