dt-bindings: timer: renesas: tmu: Document r8a774c0 bindings

Document RZ/G2E (R8A774C0) SoC in the Renesas TMU bindings.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>