Doc: hisi: hns adds mc-mac-mask property

Since there is not enough tcam table entries for every vlan and multicast
address, HNS needs to add support of fuzzy matching of TCAM tables. Adding
the property to mask the bits to be fuzzy matched, so update the bindings
document

Signed-off-by: Kejian Yan <yankejian@huawei.com>
Reviewed-by: Yisen Zhuang <yisen.zhuang@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>