dt-bindings: timer: renesas, cmt: Document r8a774b1 CMT support

Document SoC specific bindings for RZ/G2N (r8a774b1) SoC.
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>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1570104229-59144-1-git-send-email-biju.das@bp.renesas.com

Documentation/devicetree/bindings/timer/renesas,cmt.txt

@@ -29,6 +29,8 @@ Required Properties:
     - "renesas,r8a77470-cmt1" for the 48-bit CMT1 device included in r8a77470.
     - "renesas,r8a774a1-cmt0" for the 32-bit CMT0 device included in r8a774a1.
     - "renesas,r8a774a1-cmt1" for the 48-bit CMT devices included in r8a774a1.
+    - "renesas,r8a774b1-cmt0" for the 32-bit CMT0 device included in r8a774b1.
+    - "renesas,r8a774b1-cmt1" for the 48-bit CMT devices included in r8a774b1.
     - "renesas,r8a774c0-cmt0" for the 32-bit CMT0 device included in r8a774c0.
     - "renesas,r8a774c0-cmt1" for the 48-bit CMT devices included in r8a774c0.
     - "renesas,r8a7790-cmt0" for the 32-bit CMT0 device included in r8a7790.