samples: boards stm32 power mgmt nucleo_wb55rg sets a lptim prescaler

configure the nucleo_wb55rg with a prescaler on the LPTIMer input
clock to increase the max sleep duration.
With 32 the input clock is 1024Hz and max reachable timeout is
64 seconds.

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