clocksource/drivers/timer-ti-dm: Make omap_dm_timer_set_load_start() static

Fix sparse warning:

drivers/clocksource/timer-ti-dm.c:589:5: warning:
 symbol 'omap_dm_timer_set_load_start' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <daniel.lezcano@linaro.org>
Link: https://lkml.kernel.org/r/20190322144302.6704-1-yuehaibing@huawei.com