clocksource/drivers/rockchip: Add err handle for rk_timer_init

Currently rockchip_timer doesn't do some basic cleanup work when
failing to init the timer. Let's add err handle routine to deal
with all the err cases.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>