soc: stm32: SEGGER RTT requires some extra configuration

On some STM32 boards, for unclear reason,
RTT feature is working with realtime update only when
  * one of the DMA is clocked
and sometimes also
  * one of the DBGMCU bit STOP/STANDBY/SLEEP is set
Fixes #34324

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