clocksource/drivers/sh_cmt: Use "kHz" for kilohertz

"K" stands for "kelvin".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200618080212.16560-1-geert+renesas@glider.be