boards: nucleo_u575zi_q: Instructions and configuration for openocd

STM32U5 support is not yet supported in upstream openocd.
Provide instructions to use STMicro openocd fork as a temporary
workaround.

Additionally, provide openocd configuration to be used for this
target.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>