clocksource: sirf: Remove unused variable

Commit ef89af1f ("clocksource: sirf: Remove hard-coded clock rate")
removes all uses of the timer_div variable. Since the variable is no
longer used it should be removed.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>