net: lwm2m: RD Client update registration timeout

Renamed LWM2M_RD_CLIENT_EVENT_REG_UPDATE_FAILURE to
LWM2M_RD_CLIENT_EVENT_REG_TIMEOUT.

Changed reported event type for registration timeout to
LWM2M_RD_CLIENT_EVENT_REG_TIMEOUT from
LWM2M_RD_CLIENT_EVENT_REGISTRATION_FAILURE.
LWM2M_RD_CLIENT_EVENT_REGISTRATION_FAILURE should be only
reported case when server reject by response registration.

Signed-off-by: Juha Heiskanen <juha.heiskanen@nordicsemi.no>