MIPS: Loongson: Rename CONFIG_LEMOTE_MACH3A to CONFIG_LOONGSON_MACH3X

Since this CONFIG option will be used for both Loongson-3A/3B machines,
and not all Loongson-3 machines are produced by Lemote, we rename
CONFIG_LEMOTE_MACH3A to CONFIG_LOONGSON_MACH3X.

Signed-off-by: Huacai Chen <chenhc@lemote.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/7190/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>