dt-bindings: timer: renesas: tmu: Document r8a774b1 bindings

Document RZ/G2N (R8A774B1) SoC in the Renesas TMU 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/1569249052-1037-1-git-send-email-biju.das@bp.renesas.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

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

@@ -11,6 +11,7 @@ Required Properties:
   - compatible: must contain one or more of the following:
     - "renesas,tmu-r8a7740" for the r8a7740 TMU
     - "renesas,tmu-r8a774a1" for the r8a774A1 TMU
+    - "renesas,tmu-r8a774b1" for the r8a774B1 TMU
     - "renesas,tmu-r8a774c0" for the r8a774C0 TMU
     - "renesas,tmu-r8a7778" for the r8a7778 TMU
     - "renesas,tmu-r8a7779" for the r8a7779 TMU