clocksource: dw_apb_timer_of: Mark a few more functions as __init

These are all only called by dw_apb_timer_init which is an __init
function, too

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>