Register separate QOM types for each mips cpu model,
so it would be possible to reuse generic CPU creation
routines.
Backports commit 41da212c9ce9482fcfd490170c2611470254f8dc from qemu
no logical change, only code movement (and fix a comment typo).
Backports commit 26aa3d9aecbb6fe9bce808a1d127191bdf3cc3d2 from qemu
Also backports commit 5502b66fc7d0bebd08b9b7017cb7e8b5261c3a2d