clk: st: Use round to closest divider flag

This patch uses CLK_DIVIDER_ROUND_CLOSEST flag to specify
the divider has to round to closest div.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>