target-m68k: Rename CPU types

In the initial conversion of CPU models to QOM types, model names were
mapped 1:1 to type names. As a side effect this gained us a type "any",
which is now a device.

To avoid "-device any" silliness and to pave the way for compiling
multiple targets into one executable, adopt a <name>-<arch>-cpu scheme.

No functional changes for -cpu arguments or -cpu ? output.

Signed-off-by: Andreas Färber <afaerber@suse.de>