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.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>