drivers/clocksource/timer-ti-dm: Don't call clk_get_rate() in stop function

clk_get_rate() might sleep, and that prevents dm-timer based PWM from being
used from atomic context.

Fix that by getting fclk rate in probe() and using a notifier in case rate
changes.

Fixes: af04aa85 ("ARM: OMAP: Move dmtimer driver out of plat-omap to drivers under clocksource")
Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1696312220-11550-1-git-send-email-ivo.g.dimitrov.75@gmail.com