clocksource: dw_apb_timer_of: add clock-handling

Add the possibility to get the clock-frequency from a timer clock instead
of specifying it as dt property. Additionally also add the possibility
to also define a controlling periphal clock for the timer block.

The clock-frequency property is kept to act as fallback if no clocks
are specified.

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