pwm: tegra: Read PWM clock source rate in driver init

It is required to know the PWM clock source frequency to calculate the
PWM period.

In driver, the clock source frequency of the PWM does not get change
and, hence, get the clock source frequency in driver init. Get this
values later for period calculation from pwm_config().

This will help in avoiding the clock call for getting clock rate in the
pwm_config() each time.

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