soc: st: warn if running with debug on sleep enabled

Add a compiler warning when building for an STM32 series MCU with PM
enabled but STM32_ENABLE_DEBUG_SLEEP_STOP=y. That option greatly
increases the power consumption during sleep, which is probably
undesirable when building with PM=y, and it can be activated silently
as a side effect of other options (namely RTT) making it tricky to find
out.

Add a warning to expose the situation.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>