boards: arm: Add RTC clock source for 96b_aerocore2

The `96b_aerocore2` board doesn't have its RTC node enabled,
and is failing the following test:
`tests/benchmarks/footprints/benchmark.kernel.footprints.pm`
Enabled RTC node and lsi clock source for that platform.

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