MIPS: Loongson64: fix typos in loongson_regs.h

Fix some symbol names to align with Loongson's User Manual wording. Also
correct the comment in csr_readq() suggesting the wrong instruction in
use.

Fixes: 6a6f9b7d ("MIPS: Loongson: Add CFUCFG&CSR support")
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>