clocksource: arm_generic: use integer math helpers

This will make the two crucial integer divisions in the generic
ARM arch timer used for ARMv8 use the kernel DIV_ROUND_CLOSEST()
helper inline from <linux/kernel.h> so they get more precise.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>