clocksource/drivers/timer-ti-dm: Do one override clock parent in prepare()

omap_dm_timer_prepare() is setting up the parent 32KHz clock. This
prepare() gets called by request_timer in the client's driver. Because of
this, the timer clock parent that is set with assigned-clock-parent is being
overwritten. So drop this default setting of parent in prepare().

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Suman Anna <s-anna@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200427172831.16546-1-lokeshvutla@ti.com