dts: arm: st: f3: add support for timer kernel clock

Add support for timer kernel clock for STM32F3.
Also reorganizes the timer instances:
- TIM3 is not available on F301, F318, F302x6 or x8, but is available for
  all others
- TIM4 is available on F302xB and higher, F303xB and higher, F358, F398,
  F373 and F378
- TIM7 is not available on F301, F318, F302, but is available for all
  others
- TIM8 is only available on F303xB and higher, F358 and F398
- TIM20 is only available on F303xD and xE,and on F398

Depending of the SoC version, some timers have access to one or two
distinct clock sources. Timers with NO_SEL selection only have access
to the base TIMPLCKx clock, those defined with TIMx_SEL(0) can use another
source clock: STM32_SRC_TIMPLLCLK. That's why some clocks are redefined.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>