MIPS: Loongson: Rename CPU device-tree binding

Loongson-3A R1/R2/R3 and Loongson-3B R1/R2 use the same package naming
in dts, and Loongson-3A R4 will be different. In cpu.h the classic 64bit
Loongson processors are called Loongson64C (C for classic, pre Loongson-
3A R4), and the new 64bit Loongson processors are called Loongson64G (G
for generic, Loongson-3A R4+). To keep consistency and make extensible,
we rename the classic "loongson3" prefix to "loongson64c", and the new
prefix for Loongson-3A R4+ will be "loongson64g".

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>