ARM: dts: renesas: r8a73a4: Fix thermal parent clock

ARM: dts: renesas: r8a73a4: Fix thermal parent clock

According to Table 8.1, "Summary of Module Power, Reset condition and
Clock assignment" of the R-Mobile APE6 Hardware Manual Rev. 0.7, the
parent clock of the thermal sensor clock is the Common Peripheral (CP)
clock, which runs at 13 MHz (main clock / 2).

As the R-Car Thermal driver does not use the clock rate, this change has
no functional impact.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/aac49d67d7a38230875543d49e84fcca587fb9e1.1705315614.git.geert+renesas@glider.be