boards: arm: Add RTC clock source for az3166_iotdevkit

The `az3166_iotdevkit` board doesn't have its RTC node enabled,
and is failing the following test:
`tests/benchmarks/footprints/benchmark.kernel.footprints.pm`
Enabled RTC node for that platform.

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