boards: arm: 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)

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