net: lwm2m: API for multiple resource writing

Used the registry lock in the functions lwm2m_registry_lock() and
lwm2m_registry_unlock() to make the registry lockable through a public
API. If writing to multiple resources that are composite-observed,
locking will halt the main thread until every resource is written to,
ensuring that only one notify message will be sent. Updated
the documentation in lwm2m to include this addition.

Signed-off-by: Ola Tangen Kulseng <ola.kulseng@nordicsemi.no>