sample: blink_led: Adjust PWM period

The maximum period a PWM controller can generate depends on its input
clock and its resolution (16-bit, 32-bit...). Setting a 1s max period
generates error with some hardwares (mimxrt1064_evk) because of clock
limitation.

This patch fixes this issue by calculating maximum period of the PWM
device to adjust max and min periods accordingly.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>