dts: bindings: 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.
Also prescaler is common to all PWM channels of the same timer.
This change also prepares the introduction of timer based counter
(which requires prescaler at timer level)
For compatibility reason temporarily keep pwm binding to avoid
breaking boards out of tree.
Block st,prescaler property in lptim binding
as lptim doesn't use this property for now.

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