clocksource: dw_apb_timer_of: select DW_APB_TIMER

dw_apb_timer_of is the driver part facing devicetree platforms and
calls into dw_apb_timer with the data gathered from the dt.

Currently the two platforms using the dw_apb_timer_of select both
the options for the core timer and the dt addon.

As dw_apb_timer_of always depends on dw_apb_timer let it select
DW_APB_TIMER itself without the need for every platform to do it.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Dinh Nguyen <dinguyen@altera.com>