dts: arm: st: h7rs: add support for timer kernel clock

Add support for timer kernel clock for STM32H7RS.

Define a new property for the timer prescaler in the RCC binding of H7RS.

Also fix the clock bus of TIM16 and TIM17 (they are on APB2 instead of 1)

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