net: lwm2m: Add dimension discovery support

Multi-instance resources shall report its dimension (number of
resource instances) on discovery. Since it was not possible to tell
simply on the instance count whether the resource is multi-instance or
not (there could be a multi-instance resource with only one instance
avaialble) add a new parameter to the structure representing resource,
indicating whether it's multi-instance or not.

Add dimension information to the discovery result.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>