pwm: meson: Improve PWM calculation precision

When using input clocks with high rates, such as clk81 (166MHz), the
fin_ns = NSEC_PER_SEC / fin_freq can introduce a significant error.

Ex: fin_freq = 166666667, NSEC_PER_SEC = 1000000000
    fin_ns = 5,9999999

which is, of course, rounded down to 5. This introduces an error of ~20%
on the period requested from the PWM.

This patch uses ps instead of ns (and 64 bit integers) to perform the
calculation. This should give a good enough precision.

Fixes: 211ed630 ("pwm: Add support for Meson PWM Controller")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

squash! pwm: meson: Improve pwm calculation precision