net: bt: enable solicit node multicast group registration

6lowpan over BLE should work without solicit node multicast messages
according to RFC7668[1], but that requires Neighbor Solicitation with
Address Registration Option, which is currently not implemented in
either Zephyr or Linux. This is causing the router to fallback to normal
neighbor solicitation based discovery, but the NS frames are being
discarded in the host stack because the solicit node multicast groups
are not registered.

This drops the NET_L2_MULTICAST_SKIP_JOIN_SOLICIT_NODE as a workaround
and adds a TODO about it.

[1] https://tools.ietf.org/html/rfc7668#section-3.2.3



Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>