pwm: sunxi: choose best prescaler to improve PWM resolution

Choose best prescaler to improve PWM resolution. Without this change
driver chooses first prescaler that gives us period value within
range, but it could be not the best one.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Tested-by: Vagrant Cascadian <vagrant@debian.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>