pwm: tegra: Increase precision in PWM rate calculation

The rate of the PWM calculated as follows:

	hz = NSEC_PER_SEC / period_ns;
 	rate = (rate + (hz / 2)) / hz;

This has the precision loss in lower PWM rate.

Change this to have more precision as:

	hz = DIV_ROUND_CLOSEST_ULL(NSEC_PER_SEC * 100, period_ns);
	rate = DIV_ROUND_CLOSEST(rate * 100, hz)

Example:

1. period_ns = 16672000, PWM clock rate is 200 KHz.

	Based on old formula
		hz = NSEC_PER_SEC / period_ns
		   = 1000000000ul/16672000
		   = 59 (59.98)
		rate = (200K + 59/2)/59 = 3390

	Based on new method:
		hz = 5998
		rate = DIV_ROUND_CLOSE(200000*100, 5998) = 3334

	If we measure the PWM signal rate, we will get more accurate
	period with rate value of 3334 instead of 3390.

2.  period_ns = 16803898, PWM clock rate is 200 KHz.

	Based on old formula:
		hz = 59, rate = 3390

	Based on new formula:
		hz = 5951, rate = 3360

	The PWM signal rate of 3360 is more near to requested period
	than 3333.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>