meta-ibm: 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 dunfell (previous).

(From meta-ibm rev: 284e2ad06544f567582b2732a22e052190b5b95a)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ic1cb74ebd387715b08feccd8b94bfd0be7d35205
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>