Added Seagate U Series X family, Seagate U8 family, and Seagate

Medalist 8641 family to knowndrives table.


git-svn-id: https://smartmontools.svn.sourceforge.net/svnroot/smartmontools/trunk@1673 4ea69e1a-61f1-4043-bf83-b5c94c648137