samples: boards: pm on stm32 boards with serial wakeup

This commit retrains the power management testcase
tests/samples/boards/stm32/power_mgmt/serial_wakeup
to bun on stm32 boards only.
Limiting first to nucleo_wg55 and stm32l562dk targets,
more to be added.

Signed-off-by: Francois Ramu <francois.ramu@st.com>