Bluetooth: Add l2cap_add_psm() and l2cap_add_scid()

The intention is to get rid of the l2cap_sk_list usage inside
l2cap_core.c. l2cap_sk_list will soon be replaced by a list that does not
depend on socket usage.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>