samples: boards: stm32: pm: s2ram: enable standby mode

Add Standby mode to the overlay since here we want to test Suspend to RAM.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>
(cherry picked from commit cfa7e38378a9023cd6b4e1bc093de8f82e4b413b)
1 file changed