clocksource: sirf: Remove hard-coded clock rate

The customers may want to adjust the whole PLL and dividers according to
different user scenerios, and this causes the parent clock of sirf clocksource
not be divided exactly by the current hard-coded 1MHz clock rate.
This patch removes the hard-coded rate and makes the clocksource driver more
adaptive to the external changes.

Signed-off-by: Yanchang Li <yl22@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>