pwm: iqs620a: Use 64-bit division

The PWM framework is going to change the PWM period and duty cycles to
be 64-bit unsigned integers. To avoid build errors on platforms that do
not natively support 64-bit division, use explicity 64-bit division.

Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>