Basic support for Firestone and Garrison systems

Firestone and Garrison are IBM branded OpenPOWER systems.
diff --git a/meta-openbmc-machines/meta-openpower/meta-ibm/meta-garrison/conf/conf-notes.txt b/meta-openbmc-machines/meta-openpower/meta-ibm/meta-garrison/conf/conf-notes.txt
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+Common targets are:
+     obmc-phosphor-image