pwm: sti: Remove PWM period table

Removes the PWM period table. Instead the prescaler is computed
from the period value passed in the config() function.

Signed-off-by: Ajit Pal Singh <ajitpal.singh@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>