drivers: pwm: stm32: move "st,prescaler" to timers instead of pwm

Prescaler was misplaced in pwm binding, instead of timers binding.
For example, TIM6/TIM7 doesn't have PWM capability,
but have a prescaler.
This change also prepares the introduction of timer based counter
(which requires prescaler at timer level)
For compatibility reason temporarily use pwm prescaler if it exists,
otehrwise use timers prescaler.
And thus avoid to avoid breaking boards out of tree.

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>