MIPS: define more Loongson CP0.Config6 and CP0.Diag feature bits

These are exposed to userland alternatively via the new CPUCFG
instruction on Loongson-3A R4 and above. Add definitions for readback
on older cores.

Signed-off-by: WANG Xuerui <git@xen0n.name>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>