cpufreq: dt: Add support for r8a7742

Add the compatible strings for supporting the generic cpufreq driver on
the Renesas RZ/G1H (R8A7742) SoC.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Marian-Cristian Rotariu <marian-cristian.rotariu.rb@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>