target-openrisc: TYPE_OPENRISC_CPU should be abstract

A basic assumption of CPU subtypes is that only specific models get
instantiated. A user is not supposed to instantiate an <arch>-cpu.
Suppress it via abstract = true, which also drops or32-cpu from
-cpu ? output.

Cc: qemu-stable@nongnu.org
Cc: Jia Liu <proljc@gmail.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
target-openrisc: Rename CPU subtypes

Model names were mapped 1:1 to type names. As a side effect this
registered 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>