dw_apb_timer_of.c: Remove parts that were picoxcell-specific

It seems we made a mistake when creating dw_apb_timer_of.c:
picoxcell sched_clock had parts that were not related to
dw_apb_timer, yet we moved them to dw_apb_timer_of, and tried to
use them on socfpga.

This results in system where user/system time is not measured
properly, as demonstrated by

    time dd if=/dev/urandom of=/dev/zero bs=100000 count=100

So this patch switches sched_clock to hardware that exists on both
platforms, and adds missing of_node_put() in dw_apb_timer_init().

Signed-off-by: Pavel Machek <pavel@denx.de>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>