ARM: plat-nomadik: use DIV_ROUND_CLOSEST()

Use DIV_ROUND_CLOSEST() to calculate the cycle counter for the
periodic mode instead of relying on homebrew reimplementation.

Cc: Alessandro Rubini <rubini@unipv.it>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>