gpio: mvebu: improve pwm period calculation accuracy

Change 'off' register value calculation from

  $off = (period - duty_cycle) * clkrate / NSEC_PER_SEC

to

  $off = (period * clkrate / NSEC_PER_SEC) - $on

That is, divide the full period value to reduce rounding error.

Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>