pwm: tegra: Improve required rate calculation

For the case where dev_pm_opp_set_rate() is called to set the PWM clock
rate, the requested rate is calculated as ...

 required_clk_rate = (NSEC_PER_SEC / period_ns) << PWM_DUTY_WIDTH;

The above calculation may lead to rounding errors because the
NSEC_PER_SEC is divided by 'period_ns' before applying the
PWM_DUTY_WIDTH multiplication factor. For example, if the period is
45334ns, the above calculation yields a rate of 5646848Hz instead of
5646976Hz. Fix this by applying the multiplication factor before
dividing and using the DIV_ROUND_UP macro which yields the expected
result of 5646976Hz.

Fixes: 1d7796bd ("pwm: tegra: Support dynamic clock frequency configuration")
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>