Add: Software update directory for aix 6.1 powerpc

Ref: https://dev.cfengine.com/issues/6583

In order for the binary upgrade policy in `update/update_bins.cf` to
work for aix the specific aix version is needed.
Merge pull request #327 from nickanderson/redmine/6583

Add: Software update directory for aix 6.1 powerpc