pwm: rcar: Improve calculation of divider

The rcar_pwm_get_clock_division() has a loop to calculate the divider,
but the value of div should be calculatable without a loop. So, this
patch improves it.

This algorithm is suggested by Uwe Kleine-König and Laurent Pinchart.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>