tools/ynl: Add mcast-group schema parsing to ynl

            self.attr_set = self.family.attr_sets[attr_set_name]

class SpecMcastGroup(SpecElement):

    """Netlink Multicast Group

    Information about a multicast group.

    Value is only used for classic netlink families that use the

    netlink-raw schema. Genetlink families use dynamic ID allocation

    where the ids of multicast groups get resolved at runtime. Value

    will be None for genetlink families.

    Attributes:

        name      name of the mulitcast group

        value     integer id of this multicast group for netlink-raw or None

        yaml      raw spec as loaded from the spec file

    """

    def __init__(self, family, yaml):

        super().__init__(family, yaml)

        self.value = self.yaml.get('value')

class SpecFamily(SpecElement):

    """ Netlink Family Spec class.

        ntfs       dict of all async events

        consts     dict of all constants/enums

        fixed_header  string, optional name of family default fixed header struct

        mcast_groups  dict of all multicast groups (index by name)

    """

    def __init__(self, spec_path, schema_path=None, exclude_ops=None):

        with open(spec_path, "r") as stream:

        self.ops = collections.OrderedDict()

        self.ntfs = collections.OrderedDict()

        self.consts = collections.OrderedDict()

        self.mcast_groups = collections.OrderedDict()

        last_exception = None

        while len(self._resolution_list) > 0:

    def new_operation(self, elem, req_val, rsp_val):

        return SpecOperation(self, elem, req_val, rsp_val)

    def new_mcast_group(self, elem):

        return SpecMcastGroup(self, elem)

    def add_unresolved(self, elem):

        self._resolution_list.append(elem)

                self.ops[op.name] = op

            elif op.is_async:

                self.ntfs[op.name] = op

        mcgs = self.yaml.get('mcast-groups')

        if mcgs:

            for elem in mcgs['list']:

                mcg = self.new_mcast_group(elem)

                self.mcast_groups[elem['name']] = mcg