clocksource: dw_apb_timer_of: use clocksource_of_init

dw_apb_timer_init used to search the devicetree for matching timer
devices, making calls to it from board files necessary.

Change the dw_apb_timer_init to work with CLOCKSOURCE_OF_DECLARE.
With this change the function gets called once for each timer node
and tracks these number of calls to attach clockevent and clocksource
devices to the nodes.

Also remove the calls to dw_apb_timer_init from all previous users, as
clocksource_of_init is the default for init_time now.

Tested on the upcoming rk3066 code.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Dinh Nguyen <dinguyen@altera.com>