pwm: sun4i: Use nsecs_to_jiffies to avoid a division

Since the PWM framework is switching struct pwm_state.period's datatype
to u64, prepare for this transition by using nsecs_to_jiffies() which
does away with the need for a division operation.

Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>