MIPS: Loongson: Add basic Loongson-3 definition

Loongson-3 is a multi-core MIPS family CPU, it support MIPS64R2 fully.
Loongson-3 has the same IMP field (0x6300) as Loongson-2.

Loongson-3 has a hardware-maintained cache, system software doesn't
need to maintain coherency.

Loongson-3A is the first revision of Loongson-3, and it is the quad-
core version of Loongson-2G. Loongson-3A has a simplified version named
Loongson-2Gq, the main difference between Loongson-3A/2Gq is 3A has two
HyperTransport controller but 2Gq has only one. HT0 is used for cross-
chip interconnection and HT1 is used to link PCI bus. Therefore, 2Gq
cannot support NUMA but 3A can. For software, Loongson-2Gq is simply
identified as Loongson-3A.

Exsisting Loongson family CPUs:
Loongson-1: Loongson-1A, Loongson-1B, they are 32-bit MIPS CPUs.
Loongson-2: Loongson-2E, Loongson-2F, Loongson-2G, they are 64-bit
            single-core MIPS CPUs.
Loongson-3: Loongson-3A(including so-called Loongson-2Gq), they are
            64-bit multi-core MIPS CPUs.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Hongliang Tao <taohl@lemote.com>
Signed-off-by: Hua Yan <yanh@lemote.com>
Tested-by: Alex Smith <alex.smith@imgtec.com>
Reviewed-by: Alex Smith <alex.smith@imgtec.com>
Cc: John Crispin <john@phrozen.org>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/6629/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>