Update documentation for STM32U5 and STM32L5 based boards:
- Zepĥyr SDK openocd can be used instead of openocd ST fork.
- All these boards use a STLinkV3 instead of V2.
- Update (or provide when missing) instructions for building and
flashing TFM applications.
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>