clk: sunxi: fix some calculations

Some divisor calculations were misrounded, causing higher than requested
rates on some clocks. Fix them up using DIV_ROUND_UP, and replace one
homebrew instance of it as well with the right macro.

Reported-by: Boris BREZILLON <b.brezillon.dev@gmail.com>
Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Mike Turquette <mturquette@linaro.org>