MIPS: generic: Enable all CPUs supported by virt board in Kconfig

Enable extra CPUs that may be supported by virt board, including
R4x00 (R4000 in QEMU), Cavium Octeon (Octeon68XX in QEMU), loongson2e,
loongson2f.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>