ARM: at91: PIT: Use DIV_ROUND_CLOSEST to compute the cycles

Until now, the pit_cycle computation was dividing the rate by HZ, rounding to
the closest integer, but without using the appropriate macro.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>