meta-mellanox: update Yocto compat for gatesgarth

The next release of Yocto is soon and most of the upstream
layers have switched support strings for it.  Support layer
compat for gatesgarth (current) and dunsfell (previous).

(From meta-mellanox rev: a2c7d70e8f7a78a205dc012643cb94cb28311bbf)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I25a711db27ca00cae6cce7609574e81626652c87
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>