MIPS: Loongson64: define offsets and known revisions for some CPUCFG features

Add the constants for easier and maintainable composition of CPUCFG
values.

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>