devlink: Add API to register packet trap groups

Currently, packet trap groups are implicitly registered by drivers upon
packet trap registration. When the traps are registered, each is
associated with a group and the group is created by devlink, if it does
not exist already.

This makes it difficult for drivers to pass additional attributes for
the groups.

Therefore, as a preparation for future patches that require passing
additional group attributes, add an API to explicitly register /
unregister these groups.

Next patches will convert existing drivers to use this API.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>