net: lwm2m: Utility functions added to LWM2M Engine

Added 3 utility functions :
- lwm2m_engine_update_period_service() which updates the period of a
given service.
- lwm2m_engine_update_period_min_observer() which updates the
min_period_sec for a given observe node.
- lwm2m_engine_update_period_max_observer() which updates the
max_period_sec for a given observe node.

Signed-off-by: Thomas LE ROUX <thomas.leroux@smile.fr>