clocksource/drivers/sh_cmt: Use of_device_get_match_data() helper

Use the existing of_device_get_match_data() helper instead of
open-coding its functionality.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>