dts: arm: st: f4: add support for timer kernel clock

Add support for timer kernel clock for STM32F4.

Define a new RCC binding to add the timer prescaler property (timpre).
This new binding is used for all STM32F4 except F405/F407/F415/F417 who do
not support it.

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