pwm: imx27: Use 64-bit division macro

Since the PWM framework is switching struct pwm_state.period's
datatype to u64, prepare for this transition by using
DIV_ROUND_UP_ULL to handle a 64-bit dividend.

Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>