soc: st: stm32: build warning for `STM32_ENABLE_DEBUG_SLEEP_STOP`

Add a warning in the build system if both `CONFIG_PM` and
`STM32_ENABLE_DEBUG_SLEEP_STOP` are enabled at the same time. The first
is likely only enabled if the SoC is intended to be driven into low
power states to save power, while the later prevents the SoC from being
as low power as it can be.

Signed-off-by: Jordan Yates <jordan@embeint.com>