clocksource:sirf: remove the hardcode for the clk of timers

Nobody want to know the connection between io clk and timer clk,
so exposing this information to timer module is not reasonable.
this patch moves to define the timers' clk in dt.

Signed-off-by: Zhiwu Song <Zhiwu.Song@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>