soc: st: common: Enable debug in sleep mode on H7

Similarly to debug in stop mode, enable debug in sleep mode on H7 series,
as initially described by `STM32_ENABLE_DEBUG_SLEEP_STOP` option.

Not extending it further to other series as I won't be able to test.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>

# Conflicts:
#	soc/st/stm32/common/soc_config.c