net: lwm2m: Add default firmware update port

If a firmware update is fetched from a server, and no port number is set
in the URI (e.g. coap://example.com/fw_update), the client will try
to connect on the port specified by CONFIG_LWM2M_PEER_PORT.
If a port different from the peer port is to be used for firmware
update, this has to be set explicitly in the URI:
coap://example.com:5683/fw_update

.

This fix adds CONFIG_LWM2M_FIRMWARE_PORT, which will be used when
fetching a firmware update without specifying the port number in the
URI.

Signed-off-by: Tjerand Bjornsen <tjerand.bjornsen@nordicsemi.no>