clk: versatile: Staticize clk_sp810_timerclken_of_get

clk_sp810_timerclken_of_get is used only in this file. Make it static.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>