cpufreq: dt: Add support for r8a7792

Add the compatible string for supporting the generic cpufreq driver on
the Renesas R-Car V2H (r8a7792) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>